Dimensional modeling – architecture and terminology
by Joakim Dalby (danish Dimensionel modellering, Datamodellering, Data varehus)
Information has become a major asset for any organization. The book Das Kapital by Karl Marx could today get the title Die Information. A data warehouse is the way to provide information to the business users and add value to the business and reuse data in a new way to increase the revenue. A data warehouse is a separated system so an user query and analysis will not slow down and not reduce the workload on the operational systems.
Data warehouse is
· Subject-oriented because collect business data from a variety of source systems.
· Integrated because using common and consistent business names and units.
· Time-variant because historical data is kept as current data at any given time.
· Non-volatile because data do not change or historical data will never be altered.
· Enrichment and processing of data for providing new knowledge.
· Collection of data in support of management's decision making process and analyzing the business with business performance measurement qualifiers and KPI Key Performance Indicator used as a measure of how a company is doing.
· Utilizing dimensional modeling, end users and analysts can easily understand and navigate the data structure and fully exploit the data for self-service BI.
· Goal to support slice and dice of data for business analysis.
· Compliance requirement for provide proof that the reported numbers are accurate and complete by having an audit trail for reconciliation check etc.
· Driven by business requirements specification and use story from stakeholders and users. A data warehouse must have focus on the needs of the business.
Data from different operational source legacy systems are stored in tables in a relational database called a data warehouse. Data will be merged and integrated inside a data warehouse to get a consistent format and use same naming. It is called conforming when create a conform column with a single and consistently labeled name that has the same common meaning with identical contents all over the data warehouse e.g. same spelling and unit of measurement. For example, product data can be found in multiple source legacy systems with different name of columns and product segmentations, therefore a data warehouse will unite, interconnect, compile, conforme and consolidate the data into one table to make it easy for the business users to make a list of all products.
It is important to design a data warehouse to support reporting and data analyses by a single and common data model that is easy to navigate in and a business user don’t think of data across of source legacy systems. Data integration become seamlessly. Many operational source legacy systems update its data to reflect the most current and recent state while a data warehouse also maintain history. For example, an ambassador who has lived in Washington, London and New Delhi for the last 25 years and always bought her perfume from a Lancôme store in Paris with an IT system that only stores current shipping address which means that sales over 25 years will be placed in India, while a data warehouse will remember all cities and give a correct picture of the sale shipped around the world.
Data mart is a specific subject area contains the data and information that is relevant for a business user. A data mart has one purpose and is customized and/or summarized data and tailored to support the specific analytical requirements of a business unit or function. It utilizes a common enterprise view of strategic data and provides business units more flexibility, control and responsibility. For example, Sales mart, Customer mart, CRM mart, Churn prediction mart, Market mart, Production mart, Inventory mart, Shipment mart, HR mart, Tax mart, Fraud detection mart etc. A data warehouse can have multiple data marts bound together with conformed and shared dimensions. A conformed dimension is shared by multiple facts that has conformed measures with same calculation methods and common unit of measure e.g. sales revenue and support revenue in two facts are both pre-VAT (value added tax) in dollar and adding them will result in a total revenue.
Dimensional modeling is a technique approach that seeks to present data in a standard, intuitive framework of dimension (descriptive context and hierarchy) and fact (measurement) and focuses on ease of end user accessibility and provides a high level of performance access to the data warehouse. Ralph Kimball recommends in the design of a data warehouse to decide what business process(es) to model by combining an understanding of the business requirements with an understanding of the available data.
Business Intelligence (BI) system provides the information that management needs to make good business decisions and data warehousing is a method to help database designer to build a comprehensive and reliable data warehouse system, e.g. based on Dimensional modeling principles. An OLAP (OnLine Analytical Processing) cube can be at the top of the Dimensional modeling to present data in tools like Excel, Power BI, QlikView, Tableau or Targit.
BI is going from data and information to knowledge and wisdom to the users.
Previously called Decision Support System. BI could also stand for Business Insight.
In the article I will focus on the basic concepts, terminology and architecture in Dimensional modeling. My homepage has other articles of how to design and implement a data warehouse system with levels of data in the ETL process (Extracting, Transformation, Loading). I see Dimensional modeling as a subset of the Entity Relationship (ER) data modeling design method of a relational database system OLTP (OnLine Transaction Processing). Peter Chen, the father of ER modeling said in his seminal paper in 1976: »The entity-relationship model adopts the more natural view that the real world consists of entities and relationships. It incorporates some of the important semantic information about the real world.« Where ER data models have little or no redundant data, dimensional models typically have a large amount of redundant data. Where ER data models have to assemble data from numerous tables to produce anything of great use, dimensional models store the bulk of their data in the single fact table or a small number of them and divide data into different databases called data mart.
Back in the 1970's William H. Inmon began to define the term data warehouse as a one part of the overall business intelligence system. An enterprise has one data warehouse with entities in a relational model as a centralized repository, and multiple data marts which source their information from the data warehouse. In the data warehouse, data is stored in 3rd normal form. Data warehouse is at the center of the Corporate Information Factory (CIF), which provides a logical framework for delivering business intelligence. A data mart is a simple form of a data warehouse that is focused on a single subject. Denormalization and redundancy is the norm for data modeling techniques in a data mart. Ralph Kimball: »A data warehouse is an union of all its data marts.« Kimball’s data warehousing architecture is also known as Enterprise Data Warehouse Bus Architecture matrix (BUS matrix) as a collection of conformed dimensions that has the same meaning to every fact.
A table contains columns also known as attributes or fields. Important tip by naming columns, do not use possessive nouns such as Recipient's birth date, better use CamelCase like RecipientBirthDate in a column name, and don’t use values in column name like EmployeeDay/NightCode better with EmployeeShiftTypeCode. With conforming or conformance we avoid synonyms columns with same content but different name and we avoid homonyms columns with same name but different content, e.g. Type is a general term better give a column a name as ProductType, CustomerType, TransactionType etc.
A column contains an atomic value like a First name or a composite value like a Full name that can be further divided into three columns for Firstname, Middlename and Surname. A null represents a value for a column that is missing at the present time or is not defined for the row.
A derived column represents a value that is derivable from the value of a related column or set of columns and not necessarily in the same table. For example, the age of a person is derivable from the date of birth column and date of today.
A single-valued column holds a single value for an entity occurrence.
A multi-valued column holds multiple values, MovieCategory »Children, Comedy«.
A composite primary key is composed of multiple columns, also called compound.
An entity is a set of objects with the same properties and is implemented as a table with rows where a primary key is unique and for use in identification of rows.
A relationship is a meaningful association among entities between primary key and foreign key in an one-to-one, one-to-many or many-to-many relationship to make sure of the consistency in database. When a database is consistent we can navigate with a join among tables and relationships become navigation paths.
SQL stands for Structured Query Language e.g. Select From Where Inner Join.
Data is a plural of datum e.g. 5.5 or 38 and after labelling them shoe size in US and in EU they become information. When I know many of my friends' shoe sizes together with other information I have knowledge that I can use for better birthday gifts or sell to a marketing campaign.
1.1. Data layer in a dimensional modeling architecture
A description of the content of data layers of the throughput from source legacy systems to business users PC screen for improved analytic decision making.
Input data area - IDA
From source legacy systems to Input data area for storage of raw data. Also called Raw data, Source, Legacy, Capture Operational Data layer, Data Acquisition or Extract. Empty this area in the beginning of the ETL process. Datatypes will be adjusted to fit with the receiving database system. Do reconciling between data warehouse and source legacy systems with reconciliation of row count and sum of values and mark as reconciled and do auditing report. (to reconcile in danish at afstemme, stemmer overens). Maybe a receipt system where IDA tells the source legacy systems that data has been received. Data might be wrong in input data area or in archive and later source legacy system will resend a new revision of data, therefore important to create output views that filter wrong data away and making correct data as an extraction to the next layer of the data warehouse. Each table has a column IdaInsertTime so all rows in all tables will use the same capture time as a received date that can be used later for keep history. IdaInsertTime represents a snapshot of the total amount of data as it look likes in the source legacy systems. IdaInsertTime can be set in a System database when the fetching is starting and ETL jobs/packages/transformations/sql will reuse the same time for all data rows. Sometimes a source legacy system has a Changed column with data/time of first inserted or data/time of latest updated data in the row/record that can be used for later history.
Exchanging data from source legacy system to IDA can be in different formats e.g. JSON data-interchange format where I prefer storing data for IDA in a relational database with tables with rows, columns and data types, I must parse a json format.
Kimball recommends that source legacy systems express data at the lowest detail possible for maximum flexibility and extensibility to provide the data warehouse for simplicity and accessibility and it will be the data warehouse to make summary data, not the operational source data system. IDA is a mirror of the source.
Archive - ARC
From Input data area to Archive. Also called Operational Data Store (ODS), Persistent Staging Area (PSA), Persistence Layer, Data Repository or History. Never empty this area because it is archiving of time variant source data and it will retain historical value changes in the source legacy system. Simple data adjustment can be done to gain same date and amount format and same data representation of a social security number etc. but it will be in new columns so the original values are unchanged. Each table has columns e.g. IdaInsertTime, ArcInsertTime (datetime2(7) default sysdatetime(), not unique per row), ArcTs (timestamp, rowversion, unique per row), ArcRecordId (a sequence number per row per table for uniqueness) to fetch delta data out of the archive that have not yet been loaded, and maybe an ArcGlobalId (unique sequence number per row across all tables). Each table can also have a source legacy system indicator. Reasons for having an archive can come from issues of data quality and data governance and external regulations e.g. Sarbanes-Oxley, Basel I, Basel II, HIPAA. Data Vault is one approach of an implementation of an archive as a back-room. Archive is a versioning of the source. Read more about how to load data from an archive in chapter 8.
Data staging area - DSA
From Archive to Data staging area by extraction to prepare data cleansing, data cleaning and data scrubbing (e.g. trim string, unicode string, max length of string, null gets a value as 0 or empty string '', replace illegal characters, replace value, verification of data type, data syntax, date format (e.g. mm/dd/yy convert to UTC yyyy-mm-dd), correcting misspelling, fix impossible values, punctuation and spelling differences), to achieve common format, notation, representation, validate data, correct data and data deduplication by selecting one of the duplicate rows and keep the others in a variety table. The data cleansing and data integration process with multiple source legacy systems is to make data cleaned and conformed, e.g. a gender code from one source as »Man, Women«, another as »M, F« will be conformed to »Male, Female« through a mapping of source data. Data enrichment according to business rules, identify dimension data and fact data with derived values e.g. total cost and revenue, elapsed time and overdue time. This is the transformation of data. Empty this area in the beginning of the ETL process because data is only transient in this layer. When an Archive is stored on an another server it is common to load data from the archive into a data staging area e.g. data to a dimension of type 2 or 7 and from the stage table do the merge or insert/update to the dimension table in the data mart area. I like to design a stage table to suit the destination or target table structure in a data mart rather than the source e.g. translate names of columns and data types, merge or divide columns like a Name to become two columns of Firstname and Surname. I like to use computed columns in a stage table for calculation, string manipulation and hashbyte value for a comparison column to compare data with a dimension table in the data mart area, but a hashed column is not good for an index because new values never comes in the end so it takes extra performance to update the index with hashbyte values.
DSA will perform a data quality and filter wrong data and invalid data into a quality assurance and quality control (QA/QC) database. DSA continues reconciling data from the archive for referential integrity i.e. foreign key value exists as primary key value, relationship cardinality rules for dependencies as mandatory, optional or contingent to make sure there is a foreign key or not, it can be null or it needs a value. Many other value checks and summation and so on for a true validation of data. When all tables in this area is loaded with correct data, the DSA is successfully completed with ValidFrom and ValidTo based on the date of today, or the source legacy system new date or changed date or the IdaInsertTime that represent a batch of insertion and mark all data to that batch time no matter which source legacy system data is coming from. All ValidFrom will be using the same batch time which is usefull for join between tables and between source legacy systems to fetch the right data shapshot at a particular time back in history.
Furthermore is derived columns and calculations ready for loading to the next layer of the data warehouse. In case of an error in the ETL process or in important data there will be raised a halt condition to stop the ETL process. Other times data will pass by with a flag in an audit dimension. Kimball has called it data wrangling to lasso the data and get it under control (data munging). DSA can also be called Work area or just Staging or STG. (In danish this layer is called data forberedelsesområde eller data rangeringsområde eller tilrettelagt lag eller behandlet lag, data vask, data udsøgning, data berigelse, data behandling, data beregning, data sammenlægning, sammenstilling, samkøring, data er blevet vasket og strøget.) Kimball calls this layer ETL-system.
Data mart - DMA
From Data staging area to Data mart with dimensional modeling, conformed and shared dimension tables, star schema around each fact table, assigned surrogate key (artificial key, identity column, an unique sequence number) for each dimension and use it in fact as foreign key. When multiple fact tables share dimensions it is called constallation schema or multi-star schema. Keep historical data in dimension and fact with ValidFrom and ValidTo columns. Make it easy for users to search for current data through a view and to give a historical date to a table-valued function to fetch dimension data and fact data at any point of time. A data mart can have multiple fact tables with different granularities. One or multiple fact tables can create an extra derived fact table with special calculations and search filter criteria to enrich data to match the business requirements specification. Never empty this area and always backup before the ETL process. Data mart is a front-room for publishing the organization’s data assets to effectively supports improved business decision making. A data mart contains one data area for one purpose and is subject-oriented therefore a data warehouse will consist of many data marts and they will have some common dimensions. To avoid to maintain the same dimension in multiple data marts may it be considered to have a dimension data mart that share its tables through views inside the other data marts. Kimball calls this layer dimensional presentation area and he is using the term first-level for a data mart contains data from only one source legacy system and second-level or consolidated for a data mart with multiple sources to cross business processes.
Presentation interface - PIF
From Data mart to Presentation interface through a data access tool or a data visualization tool like Tableau, QlikView/Qlik Sense and Power BI can import data from a dimensional schema and handle in-memory calculated KPI as quick measure using filter. An OLAP cube is loading data from a data mart and the processing do calculation of KPI and display a pivot in Excel, Targit or Power BI. A fixed report with criteria parameter do search, select and calculate data from a data mart based on an ad hoc query of a user or the report is send out as pdf file og excel file to business users every day, week, month and so on. Data mining and machine learning will also use data from a data mart. Percentage (fraction) and ratio based on a dimension slice will be calculated in the tool and a fact table contains a numerator and a denominator, e.g.:
Average price = Sum(Unit price x Quantity) / Sum(Quantity).
Active customers =
VAR measurevalue =
FILTER ('Fact_Sales';'Fact_Sales'[EndDate] = BLANK() &&
'Fact_Sales'[Amount] > 0))
RETURN IF (ISBLANK(measurevalue); 0; measurevalue)
Kimball calls this layer Business Intelligence Application.
Supporting databases (not part of data layer and is introduced by Joakim Dalby)
When custom data in the Usage database is going to be changed it can be done in a UAT user acceptance testing environment. After the ETL process has been executed we can test the data warehouse. Later we can take a backup of the Usage database and restore it in the production environment.
1.2. Data capture or data ingestion
Data capture (in danish modtagelse, hjemtagelse) from source legacy systems is based on two data flow directions:
Granularity of data capture integration strategy is to consider when the amount of data is huge in the source legacy systems. A data warehouse prefer to receive at the lowest granularity level of detail in case of specific analysis usage or data validation, but sometimes it is necessary to aggregate and summarize source data to a higher granularity like per day and per customer segment for transaction data or transactional data from an OLTP system e.g. orders, invoices, billings, payments, site hits. Non-transactional data e.g. Customer, Location, Contact, Supplier, Part, Product can be stored in a Master Data Management (MDM) database as master tables that is shared with multiple operational applications in the organization or in the company. MDM is an extension of the Usage supporting database.
Data capture from source legacy systems is based on multiple data delivery forms:
Data capture from source legacy systems is based on two data delivery methods:
It is important to do a log of number of rows/records from the source legacy systems to the Input data area for later reconciliation check count auditing (in danish kontroloptælling, afstemning) in case a source legacy system does not deliver the expected number of rows and to monitor over time the number of rows to see if it increases as expected and be used in a graph over the amount of data in the data warehouse. Other data values e.g. amount, quantity and volume can also be logged and monitored. I recommend that source legacy system tells the number of rows per table that will be compared to the saved number of rows in Input data area in an audit trail. It can be implemented as a view in the source legacy system like this:
CREATE VIEW BI_Audit_NumberOfRows AS
SELECT 'Northwind' AS Sourcename, 'Customers' AS Tablename, COUNT(*) AS NumberOfRows
SELECT 'Northwind' AS Sourcename, 'Orders' AS Tablename, COUNT(*) AS NumberOfRows
SELECT 'Northwind' AS Sourcename, 'Order Details' AS Tablename, COUNT(*) AS NumberOfRows
FROM dbo.[Order Details]
The data warehouse can also do its own delta data detection between Input data area and Archive to identify new data, changed data and deleted data to maintain historical data with ValidFrom and ValidTo datetimestamps is handled by following actions:
Sometimes the source legacy system provides a flag information like N for new data, C for changed data and D for deleted data.
I seen pair as: New – Changed, Created – Modified or Insert – Update.
Sometimes the source legacy system update a data row multiple times but only the most recent version goes to the data warehouse. If source legacy system insert and update a row before a new load to data warehouse, only the updated version goes to data warehouse. If source legacy system insert a new row and delete the same row before a new load to data warehouse, the data will never goes to data warehouse. It is seldome that a source legacy system has a log and is logging all changes with a revision nummer or a ValidFrom and ValidTo datetimestamps, therefore a data warehouse does not contains 100% full historical updates of data in an organization. Sometimes a source legacy system has for each (or selected) tables in the database an extra historical log table (shadow table) that contains all inserted, updated and deleted data rows from the original table together with an Action column (values for Insert, Update, Delete) and an EntryDate datetimestamp column that mark when the action was occurred and a SequenceId that is used for delta data detection to pull data from Source log to Input data area. An example of a source historical log table with rows every time a customer change:
Implemented as after-insert/update/delete triggers in source legacy system.
Data latency describes how quickly source data has to be ready in the data warehouse for the business users to do their reporting. Deadline is normally in the morning based on a periodic batch of source data from yesterday. For a real-time load to a data warehouse with constantly new presentation in a dashboard, the ETL process must be streaming oriented where source data continuously flows into the data warehouse do to transformation and make conforming data.
1.3. Enterprise Data Warehouse modeling architecture
In a data warehouse architecture and environment the data from the source legacy systems can go into a database called Enterprise Data Warehouse (EDW). When a EDW is modeled after Bill Inmon it is based on Peter Chen’s Entity Relationship data modeling with super-sub entity, associative entity and with 80% normalized data. The EDW offers integrated, granular, historical and stable data that has not yet been modified for a concrete usage and can therefore be seen as neutral. It can keep historical data meaning all the changes to the data or only the days-end status e.g. End Of The Day for each data revision from Archive or Input data area. An entity Person can have an one-to-many relationship to the addresses of the person’s living places in another PersonAddress entity with ValidFrom and ValidTo columns and these entities or tables will be merged into one type 2 dimension table according to Kimball in a data mart. Let the design of the data model for EDW do a »helicopter perspective« when identification of entities, e.g. salesperson or engineer will be stored in an Employee entity with an one-to-many to a Jobfunction entity or more abstraction in a Person entity where customers users can be stored too. In the EDW keep the source data with timespan columns like Start date and Stop date, and sometimes also convert the source data to row wise entity as a kind of transactional data that is a sequence of information exchange like financial, logistical or work-related data, involving everything from a purchase order that become an invoice with multiple invoice line items, to shipping status, to employee hours worked, plan and activity records, for subscription period and to insurance costs and claims. The EDW will be the source to one or multiple data marts using Dimensional modeling with denormalized data controlled by the ETL process. Or maybe the data mart is using a different modeling that fit better for use of data.
EDW can use an Anchor modeling where an anchor table is a surrogate key generator of the primary key (or operational key) from a source legacy system, called a business key. An attribute table has descriptive values from a source legacy system connected to the business key therefore attribute has a foreign key reference back to the anchor. The attribute table is based on type 2 historic, therefore it has also a surrogate key. A view will mix anchor and attribute and it become a type 7 dimension because surrogate key of anchor become a durable key and surrogate key of attribute become a type 2 surrogate key. Both keys will be added to a fact, and the fact can join to a most recent dimension to show the current value or join to a historical dimension to show the registered value when the fact data occurred with its transaction date. The model in Power BI can choose the content for its presentation interface because all data is on a silver platter. Anchor modeling for a database has also a knot that is a lookup table with basic data and a tie that is a relationship table. (»Tie the knot« means get married.) Anchor modeling has only extension of new tables and none modification of existing tables, therefore it is an agile modeling. When a business key exists in multiple source legacy systems, there will be one common anchor table and several attribute tables, because each source legacy system has its own attribute table. Changes in an agile data warehouse environment only require extensions, not modifications to the data warehouse and it become quick-moving to act quickly to easily customize the data warehouse when business changes. Agile development is performed in an iterative, incremental and collaborative manner.
There is also a Party modeling. In the next I will give a briefly summary of a new model for EDW.
When EDW is modeled after Dan Linstedt it is based on Data vault modeling that holds on historical and time-variant raw data in entities/tables with labels:
Hub (blue) separates the business keys from the rest of the model and can translate business key and/or natural key to a hashbyte e.g. ProductHashKey with business key ProductNumber. An integration point of business keys.
Link (green) integrate and store relationship between business keys and/or hub and link do not carry any data. A link has foreign keys to 2 or more hubs e.g. LinkInvoice has CustomerHashKey and ProductHashKey and its own primary key InvoiceHashKey. A link handling of one-to-one, one-to-many and many-to-many relationships because for data vault there is only many-to-many.
Satellites (yellow) store the context, descriptive data and measure values in columns of either the hubs or links e.g. a SatInvoice with primary key/foreign key InvoiceHashKey and data in InvoiceDate or SatProduct with ProductHashKey and data in ProductName. Satellites has all the relevant data for the data warehouse.
Data warehouse is using a surrogate key instead of a business key (or natural key, see more in section 4.2) to remove dependence from the source legacy system. Data vault 2.0 recommends to hash a business key value instead of using an identity column as an unique sequence number because:
· HASHBYTES('MD5',CONCAT(SSN,';',UPPER(TRIM(Area)))) to binary(16)/char(32)
· Hashing two values could create same hashkey value which is called a collision and is very unlikely else use SHA2_256.
· It is ease of copying data from environment to another like dev to test to prod without worrying about sequences being out of sync.
· When two source legacy systems share same business key (natural key) but in one source it is an integer data type and in the other source it is a string, by hashing they become same data type in the hub table and sat table.
· When a source legacy system is replaced, the new source comes with different data types e.g. from an integer business key to a string business key, it will still be hashed to same data type in the data vault, therefore no maintenance.
· A composite business key of multiple columns become one hashkey column which is easy to use for join and merge.
· Easy to create a hashkey of a business key value when needed by an ETL tool or in a sql select statement instead of doing a lookup to find the sequence number stored in a table. But of course still need to make sure that the same business key is not inserted twice in same hub table.
· Loading to tables can be done parallelly e.g. at same time to a hub table and to a satellite table from the same source legacy system table.
· For EU GDPR General Data Protection Regulation (Persondataforordning) a business key like a social security number needs to be anonymous after some year by adding a calculate cross sum of the ssn and hash it. It will be a nice scramble or encryption by a scrambling algorithm. To be sure of total anonymity only save Birthday and Gender of a person; no name, no address, no zipcode and no city. A Master Data Management (MDM) database can contain social security number, name, address together with CustomerId that is used in emails to customers. Therefore we have a surrogate key CustomerInternalId that is used in all other operational systems and data warehouse together with Birthday and Gender which is not personally identifiable or enforceable. A web service will provide all systems the data of personally from MDM database. When a customer is going to be deleted because it is his/her wish or the data limitation period, we only need to delete data in the MDM database and we can keep CustomerInternalId, Birthday and Gender in the other systems to make sure statistics remain unchanged back in time. When a system is calling the web service it will give back unknown for ssn, name and address when a customer no longer exists in MDM database.
Be aware it can be a performance issue to have columns of data type binary(16) or char(32) for storing millions of rows and for join many tables together compared by using an integer data type. For business rules the real business key values are used for definitions. This is purely for readability and maintainability.
Querying a data vault required many more joins than Inmons model. A data vault is an archive and therefore the business rules and transformations is first enforced when data is going to data marts.
Data Vault 2.0 has also Same-as link (sal which maps different business keys from multiple source system where each source has its own satellite), Point-in-time (pit which is calculated by ETL from hub/link to improve the performance of queries) and several other types e.g. Date table is called a nonhistory reference table.
RDV is a raw data vault like an archive and BDV is a business data vault.
EDM = Enterprise Data Model is an term from IBM back in 1998 for a data modeling technique for data warehousing, where ETL was called Capture, Transform, Apply.
Ralph Kimball do not like to store data in an EDW, he only store data in data marts that is using Dimensional modeling and Star schema, therefore EDW becomes an union of all data marts.
Three business intelligence enterprise data warehouse modeling architectures
Dimensional modeling ends up in a star schema or constallation schema (multi-star schema) with fact tables (analysis variable, measures, events) surrounded by dimension tables (context), where dimensions explain the facts. Dimension and fact conformance is a must in a successfull data warehouse implementation to meets the requirements of legislation, accepted practices, prescribed rules and regulations, specified standards and terms of a contract.
Before I used the general word data mart for a database with tables of dimensions and facts, but there is a more narrow word Multidimensional database (MDB) that is the source database for an OnLine Analytical Processing (OLAP) application like a cube also known as a Multidimensional online analytical processing (MOLAP) application. To me, it is a principle for a cube that all dimensions and all measures can be combined freely else divide into multiple cubes.
After data capture, please remember to implement a Reconciliation Summary Report with the results from your different recon e.g. Uncategorized assets, Invalid country codes, Derivatives transactions with missing currency code.
Audit trail (in danish kontrolspor) becomes important for the credibility of a data warehouse. An example from a source legacy system that has a value 0.003589 and export it to a txt file where the value becomes 3.589E-3 and by a mistake in the ETL process the data warehouse saved and displayed the value as 3.589. A contract number 700002848572 become 7.00003E+11 and the last part of the value got lost. When reconciliation is built-in the data model and the ETL process, this mistake would be reported and the programmer can fix the import and update his data profiling documentation.
1.6. Data quality
Some data quality skills and data cleansing examples:
1.7. Big data
Some properties of big data:
A data lake can be used for massive quantities of unstructured data and big data with tools that can easily interface with them for analysis for business insights. A datum in a lake has tags to give it a characteristic and by the tags we can fetch data from the lake without knowing the physical location like a server url with a folder path. A data lake can contain files on multiple servers on premise in different folders and in the cloud (many nodes), and we only using a tag to find and fetch data. For example, I like to find photos of smiling employees in all albums, I can search for a tag FacialExpression = smiling. A data lake is using ELT (extract, load, and then transform). A tool for a data lake can be like Apache Hadoop or Microsoft Azure. Data Discovery Area is an end-user sandbox. Can use U-SQL to dive in the data lake and fetch the wanted data.
A relational database is characteristic by ACID (Atomicity, Consistency, Isolation, Durability). This means that a transaction is either carried out completely or not at all (Atomic), that only valid data is added to the database (Consistent), that transactions never affect each other (Isolated), and that transactions are never lost (Durable). Read more.
A NoSQL = Not Only SQL database means it can use SQL type query language, but usually do not do so. NoSQL database often designed to run on clusters, made by open source and the database do not operate with a fixed schema structure but allow the addition of data without a pre-defined structure. A NoSQL database is characteristic by BASE (Basic Availability, Soft state, Eventually consistent). An ACID system guarantees data consistency after each transaction; a BASE system guarantees data consistency within a reasonable period of time after each transaction. In other words, there is data consistency in the system, just not immediately. This leads on to the Soft State principle. If the data is not consistent at all times, the system must take a temporary data state into account. The sum of both these principles means that data accessibility is given very high priority, even if coincident errors occur in the database system, operating system or hardware. If parts of the database do not work, other parts of the database take over, so that data can always be accessed.
2. Grain (granularity) of data warehouse
The grain is the level of detail of data in the table, both in dimension tables by the hierarchy and in fact tables by the definition of the measurement event and of interest, and the grain can later be expressed in terms of the dimensions. The lowest grain keep all relevant data from source legacy systems and is therefore the most flexible approach but also take most storage space and easy can cost high query performance. To improve query performances the grain can be lifted up to higher level while data will be aggregated and summarized and therefore take less storage space. Data can also be divided such as current year data is in grain daily (dately), previous year in weekly and older data in monthly, because very detailed information is normally not relevant for analysis years back in time. Granularity of fact table can be divided into three types:
Grain yearly to monthly to weekly to daily we say that each level e.g. daily increase the granularity and the number of rows in the fact table.
Aggregation is the process of calculating summary data from detail base level table rows (records) and is a powerful tool for increasing query processing speed in data marts. For example, a sale is a fact with analysis variable and measures like quantity sold and amount, dimensions like product, customer and date of purchase that bring a sale in a context. The grain of the sale is limit to a date (like December 23) and a time (like 4:30 pm) and therefore the fact is on Transaction grain. In case we drop the time, the measures would be called TotalQuantitySold and TotalAmount because they are the result of a summation of sales time to sale date and therefore the fact is on Periodic grain. Also if we decide to summarize the sales date to a weekly or montly level. In case we decide to grain the customers by aggregate them into segments and don’t keep names and adresses, then the fact becomes Accumulating/Aggregated grain. The product has a three level hierarchy of category name, brand name and product name, and therefore we can say that product has the highest level of detail.
Instead of summation of sales time or sale date and loose some important information of date of purchase, we can summarize data into two other fact tables for a weekly level for data from the last year and a monthly level for older data, because when we go back in time we do not need analyzes on a daily or weekly level and by aggregation we save harddisk space and improve the performance of the query because fewer rows in month level need to be summarized to fetch the year level data.
The lowest level of aggregation or the highest level of detail is referred as the grain of the fact table.
A fact table can contain fact’s data on detail or aggregated level depends of the grain approach. A fact table has five types of columns:
The ETL process has to secure the natural key, so fact rows are distinct. Fact tables in a dimensional model express the many-to-many relationships between dimensions and is implemented as one-to-many relationships between dimension tables and fact tables. I never have foreign key constraint on a fact table because it decrease inserting performance and I trust the ETL process and range lookup, and no human being will be doing a update or delete of a dimension table or a fact table.
3.1. Types of facts
Let us characterize the various facts into different types of facts or more exactly different types of the columns in a fact table. Conforming facts means making agreements on common business metrics such as key performance indicators (KPI) across separated source legacy systems so that these numbers can be compared mathematically for calculating differences and ratios.
Fully-Additive measure - summable across any dimension
A fact table has numerical measures that can be summed up for all of the dimensions in the fact table, so the measure columns data type is a number. A sales fact is a good example for additive fact with measures like Quantity sold and Amount. In case of a transaction dataset to a fact table refer to a measure column which value is empty, null or nullable, use the default value 0 because this won't bother aggregation like summation. Each measure must have its metrics. If it is a monetary measure, it may have a currency column and if it is an unit measure it may have a column to explain the kind of units used like centimeters, litres, cubic metres etc. Fact can have a calculated measure or a derived measure based on existing measures and constants e.g. Profit or Surplus = Revenue – Costs.
Semi-Additive measure - summable across some dimensions
A fact table has measures that can be summed up for some of the dimensions in the fact table and not for other dimensions. For example, a daily balance measure can be summed up through the customers dimension but not through the time dimension. Inventory levels cannot be summed across time periods.
Non-Additive measure - not summable for any dimension
A fact table has measures that cannot be summed up for any of the dimensions in the fact table. For example, a room temperature fact is non-additive and summing the temperature across different times of the day produces a totally non-meaningful number. However, if we do an average of several temperatures during the day, we can produce the average temperature for the day, which is a meaningful number. Other example is a percentage and a ratio measure. A fact table that only contains column with transaction number such as order number, invoice number or a voucher number that can’t be summing up. A order fact with measures like Unit price and Discount percentage where an Unit price for a single product makes no sence to summarize, but the derived column Amount = Unit price x Quantity is to be summarized and become an additive column, called a calculated measure. Trend, Stock and Ranking can't be added and in general all calculations on one specific intersection of the dimension. Year-to-Date ytd measure can’t be summed up. Count of rows is normally used.
A conformed measure in multiple facts must use the same common business rule and definition so multiple facts can be united in a report or a cube. When several data marts are using fact data with same name of fact tables or name of columns for measures and they have compatible calculation methods and units of measure and support additivity across business processes. If a measure e.g. Revenue is used in multiple fact tables with different calculations and meanings, it is best to use different column names because theses facts are not conformed.
A fact table that contains no measures is called factless or measureless fact. For example, a fact table which has only columns for employee, date, time and type, where type is like »workstart«, »workstop« and »workstopofsickness« and there are no columns for measure. You can get the number of employees working over a period by a »select count(distinct EmployeeId)« or by a distinct row-count calculated measure in your OLAP cube. For a factless fact you will normally count the number of rows, row count or counting rows and call it »Number of <a name>«. Sometimes a factless fact has a value column called Count with only one value as 1 used in a data access tool to sum over and get the number of rows. In case the fact grain is weekly and a week is missing, it can be inserted to have all weeks complete and here will Count gets the value 0. Factless fact is for registration of event or assignment i.e. attendence take place in a school class with dimensions for student, class, room and professor. If we add a measure column for Attendence with 1 or 0 per date per student it is not a factless fact anymore. Factless fact can be used to represent a many-to-many relationship among dimensions.
Capture a relationship in the fact
To be a column in a dimension or to be its own dimension and used in a fact is a good question. Kimball’s financial services example starts with an Account dimension including data of products and branches but he choose to remove these descriptive columns to form independent dimensions of Product and Branch and use them in a fact together with the Account. Therefore the fact capture a relationship among accounts, products and branches. Another example is that an account can belong to two customers and a customer can have several accounts. This many-to-many relationship can be expressed in a factless fact or in a bridge table, see later. A bank transaction is done by one customer from an account and it is natural to have a Customer dimension in the fact.
Kimball says: »Demoting the correlations between dimensions into a fact table«, and I like to add: »With capture a relationship in the fact table we also keep the registered relationship at the time the event or transaction occurred and was entered into the fact table«. An example is a retailer SuperChemi belongs to a chain Ethane at date of sale e.g. 2013-05-17 and the fact table has two columns for dimensions to Retailer and Chain. In 2015 the retailer SuperChemi changes to another chain Propane but we still keep the registered relationship back in 2013 and 2014 in the fact. When a chain is not a part of the fact table and we in year 2016 like to find sales of retailers for a specific chain e.g. Propane, we will use the current relationship between dimension Retailer and dimension Chain as a snowflake dimension, meaning that SuperChemi belongs to Propane, and when we summarize sales per year since 2010, chain Propane will include the sales of SuperChemi in 2013 even though the retailer belonged to Ethane at that time.
A fact table that describes an event or operation that occurred at a point in time in a source legacy system e.g. an invoice line item, and the data row will never be changed in the source. The row has a date e.g. a transaction date, an entry date or a post date and sometimes a time to represent the point in time with other lowest-level data. Key values for dimensions is found at transaction datetime.
Some data modeler do not like separate facts for each transaction type but build a single blended fact with a transaction type dimension and a mix of other dimensions can make N/A dimensions. I have seen a blended fact called FactEvent which I think is a poor and non-signing name of a fact table, and a date dimension gets a generalized name instead of multiple facts has names as order date, purchase date and receipt date.
Periodic snapshot fact
A fact table that describes the state of things in a particular instance of time, and usually includes more semi-additive and non-additive measures. It is a table with frozen data, meaning a row will never be changed/modified/deleted (is unchanged) because the row can have been used in a report like a monthly or annual report and later it is a must to be able to create the same raport with the exact same data. The periodic snapshot fact table will never be done empty or updated, therefore it is a true incremental load and the table has a column like a Month (yyyymm) for a monthly basis or a Year to fetch the wanted data as a month slice or a year slice. When a month or a year is over and data is ready, data will be loaded. Sometimes data for the current month is also loaded every day for a current-month-to-date, therefore the current month will be updated until it is over, finish and can be closed. Measures can be a balance in account or inventory level of products in stock and so on. Key values for dimensions is found at the end of the period. Fact table can be monthly partitioning for making a faster query performance when searching for a month.
Accumulating snapshot fact
A fact table that describes a process with milestones of multiple dates and values columns with different names which will be filled out gradually. Therefore over time the same fact table row will be revisited and updated multiple times where a default date key value is -1 for »hasn’t happened yet« or »be available later«. Each stage of the lifecycle has its own columns e.g. milestones of a hospitalization or steps of the manufacturing of a product. In a retail store a product has three movements as ordered, received and sold that would be three date dimension columns in an accumulating snapshot fact.
A fact table where a row is a summarize of measurement events occurring at predictable steps between the beginning and the end of a process. For example, a source legacy system for current payments from customers where some pay several times over a month, and the first payment become a new row in fact table with date of payment in columns BeginDate and EndDate and the amount in Paid column. The next ETL process will do a summarize of payment per customer from the Begindate to current date or end-of-month date, and then update the fact row with same BeginDate with the new summarized payment and new EndDate, so a fact row will be revisited and updated multiple times.
Kimball recommands fact data at the lowest detail grain as possible for ensures maximum flexibility and extensibility, he call it a base-level fact. A derived fact table is created for performing an advanced mathematical calculation and complex transformations on a fact table like for a specific KPI (Key Performance Indicator), or an aggregate fact with a summation of measures to a higher grain like from date level to month level and from product level to brand level and using shrunken dimensions for Month and Category as a dimension lifted to a higher grain from the base-level dimensions as Date and Product. A derived fact can be based on multiple fact tables for making faster ad hoc query performance and simplify queries for analysts and for providing a dataset to the Presentation interface.
Aggregate fact or Summarized fact
A derived fact table that is created to referred to as a pre-calculated fact with computed summarized measures at a higher grain level of one or more dimensions to reduce storage space and query time greatly and eliminate incorrect queries. An aggregate fact is derived from a base-level fact and measures in an aggregate fact is a computed summary of measures in the base-level fact. Dimensions in an aggregate fact can be derived from base-level dimensions and is called shrunken dimensions because the values is rolled up to create less fact rows e.g. Date dimension become Month dimension, Product dimension become a Category dimension and an Address dimension become a Region dimension, and therefore the measures can be summarized to less fact rows for better query performance.
Year-to-Date ytd fact where month February is a summing up or roll up of January and February and so forth. Last-year-this-year fact with calculation of index compared to last year as a new column and easy to display in a report.
Aggregate fact table is simple numeric roll up of atomic fact table data built solely to accelerate query performance. It is called incremental aggregation when an ETL process do a dynamically update of a table by applying only new or changed data without the need to empty the table and rebuild aggregates.
A fact table contents several measures but only one or few of them has a value in each fact row. For the fact rows with same dimension member repeated in multiple contiguous rows with identical values, they will be smashed or collapsed into one fact row using operation as sum, min or max to limit the number of rows in the fact.
A fact table used to combine facts from multiple source legacy systems and business processes together into a single consolidated fact table if they can be expressed at the same grain. I.e. sales actuals can be consolidated with sales forecasts in a single fact table to make the task of analyzing actuals versus forecasts simple and fast to compare how the year is going.
A fact table used for a source legacy system that is regularly updatable meaning that the source change and overwrite its values. To capture a timespan when the fact row was effective, the fact table will act as SCD type 2 dimension with a BeginAt and EndAt columns to keep historical data and to represents the span of time when the fact row was the »current truth«. It is called Slowly Changing Facts. See more in section 6.4.
Counterpart fact (negating fact) and Transactional fact
A fact table used for Slowly Changing Facts because the source legacy system is changing fact value without keeping the old value as a historical transactions. See more in section 6.4.
Column wise fact and Row wise fact
A column wise pivoted fact table is usefull to be columns in a report e.g.
Revenue Jan, Revenue Feb, Cost Jan, Cost Feb, Sales Jan, Sales Feb.
For a cube a row wise is much better because it gives good dimensions e.g.
Period, Entry, Amount.
Therefore a data warehouse needs to convert from columns to rows or vice versa.
A fact table contents huge number of rows where a period e.g. from a »date of employment« to a »termination date« as columns in one row, will be turned around to many rows with one row per day of the period, or per week or per month depends of the wanted grain of the period. When an employee has 10 year anniversary it will make more than 3650 rows on day grain, and if the employee dimension keep history of different names, departments and job functions etc. for each staff, then the fact rows will have different key references to the dimension. Another grain would be 15 minute grain which gives 96 rows per day for a windturbine power production. When the exploded fact is a source for a olap cube it can sometimes be implemented as a view in the database, and when it is for ad hoc reporting it will be used several times per day then it must be a materialized view stored in a table or sometimes as an indexed view.
3.2. Other fact classifications
Transaction has one row per transaction when they occur together with a datetime.
Periodic has one row for a group of transactions made over a period of time through summation like from daily grain to monthly grain so the Date dimension is represented by the month level with the first day of the each month. Notice that certain dimensions are not defined when compared to a transaction fact such as dimension TransactionType.
Accumulating/Aggregated has one row for the entire lifetime of an event and therefore constantly updated over time. For example, an application for a bank loan until it is accepted or rejected or a customer or working relationship. These fact tables are typically used for short-lived processes and not constant event-based processes, such as bank transactions.
Mostly a fact table describes what has happened over a period of time and is therefore an additive or cumulative facts. An example of a status column in a fact table that receive data from a school system where a student follow a course and later finish it, but sometimes a student skip the course and are delete in the system. Before reload the fact it can be a good idea to have a CourseStatus column with values like: Active, Completed or Dropped.
A dimension table containing business element contexts and the columns contain element descriptions and a dimension is referenced by multiple fact tables so the containing measurements make sense.
4.1. Purpose of a dimension
Some purposes as I seen it:
The dimension values can be placed in a hierarchy like a Location with levels Country→Region→City, and in a group or band interval like a Age with intervals of Child (0-12), Teeanage (13-19), Adult working (20-66) and Senior citizen (67-130). A dimension normally contains one or multiple hierarchies and/or groups to fulfill requirements from the users. A dimension can of course be non-hierarchical and non-grouping.
Different dimensionality covers the issue that not all combination of multiple dimension values are allowed in a fact and the data warehouse needs to make sure of the data quality.
4.2. Dimension keys
A dimension table has mininum three types of columns:
Primary key is a surrogate key identity column an unique sequence number to remove dependence from the source legacy system and for using in fact table foreign key. It can be called Entity key EK because it represent an entity from source or Surrogate key SK, but for me »surrogate« is a characteristic or a property not a name of a column. Primary key must not be data-bearing (in danish data bærende), it must be meaningless, but for a date dimension and a time dimension I like to use a smart valued primary key, e.g. a date 2013-12-31 as an integer value 20131231 and a time 08:30 am as an integer value 830 or 08:30 pm as 2030.
Business key is the primary key (or operational key) of the source legacy system, e.g. Id as a surrogate sequence number, a No (number) or a Code that is unique. Often the business key is a meaningless value e.g. integer or guid based.
Natural key is other candidate key of the source legacy system based on data in the real world, e.g. Code, Serial number or an unique Name. Natural key has an embedded meaning and represent an unique object in the business.
Textual data is representing the dimension value context description and is saved in columns of the dimension table and will be shown to the users as descriptive columns to explain fact data.
A vehicle has an unique and long serial number as natural key because the system is using an unique sequence number as primary key that become business key. In a Status table a column StatusId become business key, column StatusCode become natural key and column StatusName become textual data. In a Customer dimension table we will have columns from source legacy system like CustomerId, CustomerNumber and CustomerName together with ShippingAddress and Zipcode, Regioncode, Country, Age, Gender and what about the BillingAddress and so on. CustomerId is the business key and the Customer dimension primary key would be named e.g. Customer_key, CustomerId_dwh, CustomerEntityKey or EK or SK for surrogate key. »A beloved child has many names.«
Different source legacy systems have a natural key for a social security number per person where the business keys is an individual surrogate sequence number, therefore we can’t use the business key to interconnector (join, drill-across) the systems data instead we use the natural keys. The value of a natural key can change over time e.g. a person obtains witness protection and gets a new social security number and the business keys remains, then it is up to the ETL process to make a solid mapping. When an natural key is an employee number that is using the social security number (SSN) and the employee resign and some years later is rehired there will be two sequence numbers as business key and the data warehouse must have a way to glue or map them together so the employee only occurs once in a dimension. Another example is from a Customer source table with a identity sequence number called CustomerId as primary key and a CustomerNumber as an unique secondary index (candicate key). Sometimes a customer row is deleted by a mistake and later it will be inserted again with same CustomerNumber but for sure a new sequence number in CustomerId. That will create a problem in a data warehouse because it will over time receive the same CustomerNumber with two different values in CustomerId. When the data warehouse is only using the CustomerId as business key and not using CustomerNumber as natural key, the data warehouse will get two rows with same CustomerNumber which can give problem in reporting. Merging and integrating data can be hard to set up for the ETL process.
4.3. Changing dimensions
Source data is volatile data because they will change over time e.g. a customer change his name or address and a product change place in a hierarchical structure as a result of a reorganization. Therefore dimension values will normally be changed over time because of the volatility in source legacy systems. The rate of changes can be divided into two kinds of classifications and afterwards we will be looking into techniques to handle and tracking changes and to capture its history and preserve the life cycle of source data also called Change Data Capture CDC.
Slowly Changing Dimensions SCD
Columns of a dimension that would undergo changes over time. It depends on the business requirement whether particular column history of changes should be preserved in the data warehouse or data mart.
Rapidly Changing Dimensions RCD
A dimension column that changes frequently. If you do need to track the changes, using a standard Slowly Changing Dimensions technique can result in a huge inflation of the size of the dimension. One solution is to move the column to its own dimension with a separate foreign key in the fact table. Rapidly changing data usually indicate the presence of a business process that should be tracked as a separate dimension in a separate historical data table.
Techniques or methods to handle dimension values that is changing over time from the source legacy systems, called Ralph Kimball’s eight types or approaches:
Type 0: Original value, where the dimension value never change (method is passive) meaning keeping the original value from the source legacy system, and when the value is changed in source legacy system, we don’t change the value in the dimension. »Retain original«. A single fixed value do not change over time but can be corrected in case of an error e.g. date of birth, date of launching something or date of first purchase.
Type 1: Current value, where the old dimension value will be changed and forgotten when value is changed in source legacy system. The history of data values is lost forever. The actual, active, newest or latest of the value or most recent indicator. A fact table refers to a dimension value most recent, as-is. (In danish aktuelle, nuværende, gældende, seneste værdi). »Current by overwrite«. Section 6.2 will show an example of type 1. There is an one-to-one relationship between the business key and the surrogate key identity column as primary key of the dimension.
Type 2: Keep all values, where new dimension value is inserted into a new row to have an unlimited history of dimension values over time marked by Effective date and Expiration date (Active date, Expired date, Expiry date) (or StartDate and StopDate or BeginDate and EndDate or ValidFrom and ValidTo), a pair of data type »date« or »datetime« that represents the span of time when a value was the »current truth«. A fact table refers to a dimension value in effect when fact data occurred, as-was, often by a date column in fact table based on source data or by a current load insert date when the fact data was entered into the fact table or was created, born or occurred. (In danish oprindelige værdi). »Keep history in rows«. The value of a business key will be repeated every time the textual data is changing, therefore the primary key is a surrogate key identity column an unique sequence number. A view upon the dimension will provide the current values. A view upon the fact will provide the current keys to join to dimension view. A column called IsCurrent has two values: 0 for historical and 1 for current to mark each data row of a type 2 dimension. This is the technique for Slowly Changing Dimension. Section 6.2 will show an example of type 2. There is an one-to-many relationship between the business key and the surrogate key identity column as primary key of the dimension.
Type 3: Keep the last value, where the previous dimension value is stored in a Previous column (or Historical column) and current dimension value stored in a Current column. »Keep history in column«. This is the technique for Slowly Changing Dimension.
Type 4: Fast changing value in dimension columns will be split into one or more separate Mini Dimensions. »Keep history in tables«. This is the technique for Rapidly Changing Dimension to store all historical changes in separate historical data tables. The fact table contains foreign key for a given mini dimension.
Type 5: Builds on the type 4 Mini Dimension by embedding a current profile mini dimension key in the base dimension that’s overwritten as a type 1 Current value column. Therefore 4 + 1 = 5 type. »Sub class dimension«.
Type 6: Mixture of type 1 and type 2 columns therefore a good idea to suffix columns as _t1 and _t2 to know which columns can be overwritten in the current row. »Hybrid«. Can also have column of type 3, therefore 3 + 2 + 1 = 6 type. Type 6 act as type 2 of tracking changes by adding a new row for each new version but type 6 also updates _t1 columns on the previous row versions to reflect the current state of data by using the business key to join the new row with the previous rows.
Type 7: All rows follow type 2 to keep track of historical values with a key column and an extra key column called a durable key follow type 1 for the current value. The durable key is an integer representation of the business key. The fact table contains dual foreign keys for a given dimension (its key and its durable key) to show the historical value of the dimension at the time when the fact data was entered into the fact table or was created, born or occurred (in danish oprindelige værdi), and to show the current value of the dimension (in danish aktuelle, nuværende, gældende, seneste værdi). A view upon the dimension will provide the current values with the durable key to join to the fact durable key. »Dual Type 1 and Type 2 Dimensions«. Section 6.2 and 6.3 will show examples of type 7 dimension that has historical rows with a current mirror.
In a dimension table the columns can be a mix of type 1 and type 2 e.g. a Customer dimension where columns for customer name, street name, house or apartment number is type 1 because we only need the recent value for shipment, and columns for postal code (zip code) and city is type 2 because we like to tracking these changes and keep data for the city where a customer was living in when purchase a product. A »sales in cities« report for the last ten years will use the right city of the customer at the time the purchase occurred.
4.4. Types of dimensions
Let us characterize the various dimensions into different types of dimensions.
Conformed dimension or Shared dimension or Common dimension
A conformed dimension has the same meaning to every fact in multiple data marts and measures will be categorized and described in the same way and ensuring consistent reporting across the data warehouse. A conformed dimension is a consistent interface to make sure that data can be combined in a data warehouse and be used all over the business because values of a dimension means the same thing in each fact. With a conformed dimension we can combine and drill across from fact to fact in one data mart or over several data marts, and analyze common columns and values. Seperate fact tables can be used together with shared, common and conformed dimensions. Conforming of several source legacy system data is part of the integration to achieve a conformed dimension where data is integrated of different meanings and different columns must be compared against each other, rules must be set, and data must be cleansed to create a single version of the entity. Conformed dimensions will unite and integrate data values among different source legacy systems so it is easy to search across different types of data and sync them in a common report. Shared dimension is utilized in multiple fact tables in a data mart or across multiple data marts. Dimension values comes from either the source legacy systems or is built by business rules in Usage supporting database. Non-conformed dimension can only be used within one fact. It is part of the ETL process to do conforming by merge, unite and consolidate different source legacy system data across the enterprise for making a conformed dimension e.g. a Customer dimension from different business areas as order, sale, invoice, delivery and support service (for B2B and B2C) with both different customers and same customers and with different business key values and with different addresses like a shipping address and a billing address. Sometimes data from a conformed dimension is send back to the source legacy systems as master data to be reusable in the organization.
The dimension values can be placed in a hierarchy like a Location with three levels Country→Region→City. A dimension can have several separate and independent hierarchies with different numbers of levels.
The dimension values can be placed in a group that is grouping values in band intervals like person ages in custom buckets like a Age group column with intervals of Child (0-9), Tween (10-12), Teeanage (13-19), Young adult (20-29), Adult (30-66) and Senior citizen (67-130).
Data classification is the process of organizing data into categories, group or category is part of a categorization or grouping of data to make a dimension more user-friendly to see data of a dimension on an aggregated and summed level.
A dimension normally contains one or multiple hierarchies and/or groups to fulfill requirements from the users. A dimension can of course be non-hierarchical and non-grouping.
Date dimension or Calendar dimension
A very common dimension with the granularity of a single day with hierarchies as:
Year→Half year→Quarter→Month→Date (five levels) or
Year→Week→Date (three levels) because a week does not always belong to one month. I like to use an integer value as surrogate key identity column in format yyyymmdd e.g. 20131224 for Christmas day, therefore the fact table will contain useful information that can be used in a query like:
»between 20131201 and 20131231« for month of December 2013.
When a fact date is connected to the Date dimension it is easy to make a report based on month, quarter or yearly level or a search filter criteria as Q3 2013.
Dates or days can be grouped to a Season or TimeOfYear: Winter (December January February), Spring (March April May), Summer (June July August) and Fall (September October November) (in danish Årstider).
When a surrogate integer value is used instead of a date stamp it allows us to have values to handle these cases:
»Missing« (-1) when a source legacy system provide a date value that is null, meaning it is not present in some data, not reported, »no date«, »hasn’t happened yet« or »be available later« because the date to be determined is expected to be available later and fact table will be updated thereby. Therefore the fact table column for the date dimension gets the value -1 for missing.
»Not available« (-2) when a source legacy system provide a date value that is not known in the data warehouse e.g. date 1234-05-06. Therefore the fact table column for the date dimension gets the value -2 for unknown.
»Not applicable« (-3) when the dimension is not relevant for the fact row (N/A, N.A.).
»Bad«, »Corrupt« or »Dirty« (-4) when a source legacy system provide bad date e.g. year of birth as 2099 or 2017-02-29 but year 2017 is not a leap year.
Please read more about it in section 4.5.
A fact table has often at least one column that represent a date e.g. an entry date, order data, invoice date, shipping date. When an event is happening it has normally a date or multiple dates like an injury with an InjuryDate, a ReceivedDate at insurance company, a RulingDate, a TreatmentDate, a BillingDate and a PaymentDate. Date dimension is a role-playing dimension and therefore the data mart contains multiple views upon the date dimension so each view can join to each date column in the fact table and each view has a good name like the columns in the fact table.
Time dimension or Time-of-day dimension
A very common dimension with hierarchies such as:
Hour→Minute→Second (three levels) or
Hour→Minute (two levels)
The time dimension has the granularity of a single second with 86400 rows or of a single minute with 1440 rows.
Time dimension has an aggregation hierarchy that roll up of time periods into more summarized business-specific time grouping e.g. Time division or Time interval (»døgn inddeling«):
Morning (6 am to 8 am)
Rush hour (8 am to 11.30 am and 4 pm to 6 pm)
Lunch hour (11.30 am to 1 pm)
Afternoon hour (1 pm to 4 pm)
Dinner hour (6 pm to 8 pm)
Evening hour (8 pm to 11 pm)
Night hour (11 pm to 6 am)
I like to use an integer value as surrogate key identity column in format hhmmss e.g. 0 for 00:00:00, 63000 for 6:30:00 am and 202530 for 8:25:30 pm because 8 pm is also called 20 o’clock in the evening. When Time dimension has granularity of minute an integer value 10 stands for o'clock 00:10 and 2359 stands for 23:59. A missing time is represented with the value -1 (»no time«), not available is -2, not applicable is -3 and bad is -4 if source provide a time as 24:12:06.
In a ETL process for making fact data rows, developed in a SSIS package, the column Time_key with date type smallint will be created as a derived column to the pipeline where null value become -1 and time 10:34:45 become 1034, expression:
ISNULL(TransactionDatetime) ? (DT_I2)-1 : (DT_I2)DATEPART("hh",TransactionDatetime) * 100 + DATEPART("mi",TransactionDatetime).
All dimension members or values (or branches in the hierarchy) have the same number of levels which makes the dimension symmetrical or balanced such as the Date dimension or fixed-depth hierarchy Country→Region→City where the three levels is in separate columns. Several columns in the dimension table is not at third normal form (3NF), therefore the dimension table contains redundant data or duplicate data by nature. A regular dimension has a flattened denormalized structure. A Product dimension table can have columns like ProductId, Code, Name and other descriptive columns, a Category because products are divided into categories and each category has a Target group. When a category belongs to multiple products, the target group will be repeated, meaning it does not satisfy third normal form (3NF), but it is okay for a regular dimension.
Example of a Product dimension with hierarchy Target group→Category→Product
There is three levels in the dimension hierarchy and it is balanced meaning that each Product has a Category and each Category has a Target group, so the dimension hierarchy maximum dimensionality is 3 (level of dimensionality).
For the Product dimension the product is the lowest level of granularity, and there is two many-to-one relationships because many products roll up to a single category and many categories roll up to a single target group.
A ragged dimension has leaf members (the last level of a hierarchy) that appears at different levels of the hierarchy and therefore contains branches with varying depths and number of levels. Europe→Denmark→Copenhagen with three levels and
North America→United States→California→Sacramento with four levels representing continent→country→state→capital. A ragged dimension is implement as a Parent-child structure or with a bridge.
To keep a hierarchy structure as a fixed level balanced regular dimension I can make a dummy level for a »not applicable« state of Denmark and get this:
Denmark and France is divided into regions, Philippines is divided into provinces and Germany has states/lands, therefore the name of level »State« could be changed to a broader word like »Division« which covers the different countries' areas.
Used to model flexible hierarchical structure where some dimension values have different levels of hierarchies called unbalanced or variable-depth hierarchy. Every value in the dimension have a related parent (mother) value, except the top value. The hierarchy is asymmetrical or unbalanced because values are placed in different levels within the same hierarchy. For example, an Employee dimension where the parent is the manager and the children is the employees under the manager, and some employees are both a manager and have another manager above, and the chain of command can be different from department to department. Another example is an Organization dimension where some departments have sub-departments and some teams have sub-teams, but there are also teams that don’t have sub-teams. This is the strongest side of Parent-child dimension to modeling.
Dimensional model dimensions is comply with second normal form (2NF).
a) Splitting columns of a dimension table into smaller dimension tables with one-to-many relationships so data values fulfill and comply with 3NF and BCNF to avoid redundant data and denormalized structure. Snowflaking is normalization to 3NF.
b) Splitting a dimension hierarchy into two or more dimension tables is called »snowflake a hierarchy«. For example, a Customer dimension with the hierarchy:
Country→Region→City will be split into three dimension tables so only column City remains in the Customer dimension table that is connected to the Sales fact table, and two Snowflake dimension tables for Region and for Country with one-to-many from Country to Region and one-to-many from Region to Customer. Region and Country can be reused in other dimensions for Supplier and Store or in a FactSales to quick divide in regions and countries. Of course depending of business requirements specification.
c) Splitting a dimension with columns from different source legacy systems or when some columns will be user type-in from an application to enrich data and to avoid redundant data. See an example in section 6.1.
d) A Product dimension with a Category hierarchy is used in a Sales fact and Inventory fact. For forecasting (budgeting) the data is generated at Category level. Kimball will not do snowflaking of Product dimension instead he roll up to Category dimension as a strict subset of Product that the ETL process must take care of.
e) Splitting dimensions and move common columns to a general dimension is called an Outrigger dimension. For example, an Address dimension with street name, house or apartment number, postal code, city, county, area, region, country, gps coordinates and so forth to be used in other dimensions e.g. a Customer dimension with a shipping address and a billing address, a Building dimension with location address and an Employee dimension with home address. The Address dimension contains one row per unique address. A Sales fact can have a ShippingAddress_key and a BillingAddress_key as role-playing dimensions. Or a factless fact table with Customer_key, ShippingAddress_key, BillingAddress_key and effective and expiration dates for reporting track addresses of a customer. See more later.
f) A Shrunken dimension is not a snowflake dimension, see more later.
g) Splitting a dimension with rapidly changing columns is not snowflaking, see Mini.
h) Kimball’s financial services example starts with an Account dimension including data of products and branches but he choose to remove these descriptive columns to form independent dimensions of Product and Branch and not snowflake the Account instead add them to the fact because that’s the way business users think of them too. The fact will capture the relationship among data entities.
Some data modellers do not like Snowflake dimension because of the query performance with more tables in the join. I don’t use the terms snowflake schema and starflake schema because for me snowflake belongs to a dimension and sometimes it is great to have a denormalized structure and other times it is good to think of the above points. Most times a data mart will be a constallation schema (multi-star schema) where some dimensions is shared among different facts.
Now and then a data warehouse architecture has an extra layer after the data mart that is called presentation area where the ETL process or a view has joined and merged snowflaked dimensions together to only one dimension with all the columns with a lot of data redundancy, and that is fine since this layer will be refilled in every ETL process and it is easier for the users to do their query and for the tool to load data into its data model. Can use a materialized view. The layer is also called data consumption layer or data delivery layer for reporting, analysis and self-service BI.
Outrigger dimension or Reference dimension
When many columns belong logically together in a cluster or group it is fine to do a snowflaking to avoid a repeating large set of data and therefore making a dimension smaller and stable. Sometimes a canoe or a sailboat is using a rig to achieve balance because they are very narrow and a cluster of columns is placed in a Outrigger dimension or a reference dimension because it’s primary key will be a foreign key in the main dimension, but there is no reference found in any fact table. For example, a Product dimension has a column called LaunchDate that represent the date when the product will be available for the customers, and from that date the product can appear in the Sales fact table. The LaunchDate column is a foreign key to a Date dimension that become Outrigger dimension because Date dimension has many columns about dates, weeks, months, years and maybe fiscal columns too. Another example is demographic data for each country which is providing with 50 different columns. When we using outrigger dimensions we let each dimension has its own core columns.
Another example is from a bank with two kind of customers for Person and Company with common data as name and address and with specific data where a Person has social security number, gender and marital status and a Company has VAT identification number, industry code and turnover amount. A Customer dimension handle the connections to the fact tables and become a hub, an anchor or a party. Every time the bank gets a new customer, it will be set up in Customer dimension and the surrogate key value will be reused in either Person outrigger dimension or in Company outrigger dimension where both of them have a one-to-one relationship to the Customer dimension.
Shrunken dimension or Rollup dimension
A Shrunken dimension is a subset of another dimension columns that apply to a higher level of summary of an aggregated fact and the shrunken dimension key will appear in the fact.
a) The Month dimension is a shrunken dimension of the Date dimension. The Month dimension would be connected to a forecast fact table whose grain is at the monthly level, while Date dimension is connected to the realized fact table.
b) A base-level Sales fact has a grain per date and product and is connected to a Date dimension with columns of date, month, year and a Product dimension with columns of names of product, brand and category. The Sales fact is derived to a aggregate fact with a grain per month and category and is connected to a shunken Month dimension with columns of month, year and is connected to a shunken Category dimension with a name column. The aggregate fact has columns for Month_key and Category_key and a summary amount of the Sales fact where the ETL process is using the Product dimension to roll up a product to a category and match the category to the Category dimension. Therefore both dimensions Product and Category has a category name column that become redundant data in the dimensional modeling. Therefore shrunken is not snowflaking because a snowflake dimension is on 3 NF.
c) Shrunken roll up dimensions are required when constructing aggregate fact table. When a Sales fact has a daily grain the number of rows can become very large over time, therefore an aggregate fact summarized to monthly level will have less rows. Since the aggregate fact don’t need detailed customer information, the Customer dimension can make new shrunken dimensions for Region out of address, for Gender and for Age groups, and the summarized fact data become even less rows.
d) Sometimes users want few dimension values e.g. Red, Yellow and Green and they want Pink to become Red and Orange to become Yellow and so forth and the rest of the colours gets a residual value called Others. I will make a ColourGroup shrunken dimension with values: Red, Yellow, Green and Others. I will make a mapping table that will translate the colours e.g. Red to Red, Pink to Red, Yellow to Yellow and Orange to Yellow and the rest of the colours to Others. In the loading to the fact table I will let the colours pass by the mapping table and fetch the ColourGroup dimension to the fact table to obtain good performance for various statistics for the users.
Dimensions that represent data at different levels of granularity to give higher performance. Can also refer to hierarchies inside a dimension with a higher grain.
Like a Month dimension that is derived from a Calendar dimension or we can say that Calendar has been reduced to Month, Year and Week with the start date of the week together with a week number and which year the week belongs to. A derived dimension can also be created by aggregating two existing dimensions.
In a hospital we can from a Patient dimension and an Employee dimension derive a Person dimension. A person can over time be both an employee and a patient or at the same time when the employee become sick and will be hospitalized.
Fact data can derive dimension data and it is called a degenerate dimension.
Previously, I showed a Date dimension and a Time dimension and with a combination of them I can create a new dimension to handle date and hour to be used in Power BI Impact Bubble Chart e.g. from 2016-08-22 10:00 to 2016-08-26 22:00.
A fact table or a view can have a derived column like DateHourInterval:
DateHourInterval = FORMAT(TransactionDatetime,'dd-MM-yyyy HH:00','en-US')
A view can make the data rows for the derived dimension:
CREATE VIEW DimDateHourInterval AS
SELECT DateHourInterval = FORMAT(CAST([Date] AS datetime) +
CAST([Time] AS datetime),'dd-MM-yyyy HH:00','en-US')
FROM DimDate CROSS JOIN DimTime
WHERE Minute = 0 AND [Date] BETWEEN '2010-01-01' AND '2029-12-31'
Mini dimension or Historical dimension
For Rapidly Changing Dimensions for managing high frequency and low cardinality changes in a dimension of fast changing volatile columns they are placed in a mini dimension or historical dimension with its own surrogate key identity column which will be included in the fact table. A dimension table will be split into two tables, one with type 0 or 1 columns and the other with type 2 or 4 columns. For example, customer data with columns as Name, Address and Country→Region→City is placed in a Customer dimension table, and the fast changing columns BodyWeightAtPurchaseTime and MonthlyIncome interval e.g. $ 0-10000, 10000-25000, 25000-50000, 50000-99999 is placed in a mini dimension table called CustomerBodyWeightIncome with its own surrogate key identity column and a foreign key back to the main dimension. The sales fact table will have two columns to provide data for a customer, one key for Customer dimension and another key for CustomerBodyWeightIncome dimension. Sometimes it is necessary to have two or more mini dimensions if the columns is changing rapidly at different times. Normally there is no hierarchy in a mini dimension.
The bridge is to connect or map data that have a many-to-many relationship between a fact and a dimension or between a dimension and a dimension to take care of multivalued column at the conceptual level, and break it down at the logical level to a bridge dimension table with two one-to-many relationships and a composite primary key from the implicated data. Instead of the word Bridge table a term Helper table could be used. Peter Chen calls it an associative entity. A bridge table can also be used for ragged hierarchies. (A song can have a Verse 1, a Chorus, a Verse 2, a Chorus, a Bridge and ends with the Chorus again.)
Multivalued dimension or Many-valued dimension
a) To handle when one fact row have two or more dimension values from same dimension. For example, a Sales fact can have up to four different sales staff employees and a sales staff has many sales. Therefore we say there is a many-to-many relationship between Sales fact and Employee dimension, and sales staff employee becomes multivalued in the Sales fact. It can be implemented by an extra table called SalesEmployeeBridge that contains the Id of a fact row and the Employee_key from the Employee dimension, or the fact table will get a dimension key column for a SalesEmployeeGroup dimension meaning a group of sales staff is connected to one fact row; and since there is a many-to-many relationship between a SalesEmployeeGroup and a Employee dimension, a SalesEmployeeGroupBridge table will express that by combining the keys from SalesEmployeeGroup and Employee.
b) When buying a paint mixture the different colors are mixed with a ratio or weight (sum up to 100%) the amount of paint. One sales fact row contains many colors and one color is included in many paint mixtures. Color become multivalued and a bridge gets a weight value to tell how much of that color is used.
c) To handle when one dimension row has two or more dimension values from another dimension. For example, an employee has a group of skills and one skill can belong to several employees. Therefore we say there is a many-to-many relationship between Employee dimension and Skill dimension, and skills of an employee becomes multivalued in the Employee dimension. It can be implemented by an extra table called EmployeeSkillGroupBridge that has a composite primary key of SkillGroup_key and Skill_key, and SkillGroup_key is a foreign key in Employee dimension and Skill_key is a foreign key in Skill dimension, so the many-to-many relationship comes two one-to-many relationships. The data experience level would be placed in the EmployeeSkillGroupBridge. Sometimes we create a fact table e.g. EmployeeSkillFact, but for me a skill is a description and a characteristics of an employee in a company and facts would be a Sales fact or a Production fact. In a School mart I would place the students' courses in a fact table because it is a result of study and passed an exam. Another example is a t-shirt can have up to three sizes »small«, »medium« and »large« and they can become three columns in the T-shirt dimension, or to make a Size dimension that has a many-to-many relationship to T-shirt dimension and the relationship become a T-shirtSizeBridge with QuantityInStock. Another example is a bank account can two bank customers like wife and husband, and of course each bank customer can have several accounts like budget, saving and pension. Therefore we say there is a many-to-many relationship between Customer dimension and Account dimension. We create a BankAccount fact table that will refer to the Account dimension, and the Account dimension refer to an AccountCustomerBridge table that again refer to the Customer dimension, so the BankAccount fact table will not refer directly to the Customer dimension. The AccountCustomerBridge table contains two columns Account_key and Customer_key as a composite primary key so an account can have several customers.
Role playing dimension or Repeated dimension
A role-playing dimension is repeated two or several times in same fact i.e. a Date dimension which key column is repeated in three foreign key columns in a fact table for three roles labeled SaleDate, ShipmentDate and DeliveryDate. For each role I create a view that has distinguishable and unambiguously column names and in this example it become three views upon the Date dimension called SaleDate, ShipmentDate and DeliveryDate with columns like Date of sale, Year of sale, Date of shipment, Year of shipment, Date of delivery and Year of delivery.
A City dimension can be repeated in multiple roles in a fact table of persons like these columns: BirthplaceCity, ResidensCity, WorkingplaceCity, SeniorCity and DeathCity. It will become five views upon the City dimension.
A Manager dimension can be repeated as Sales clerk and Store manager.
Another example is a boolean dimension with key values 1/True and 0/False and in different views for several role-playing statuses the two values is translated to good texts as »Yes« and »No«, »In stock« and »Delivery made«, or »Available« and »Utilized« for a Utilization status.
When a dimension has an outrigger dimension e.g. Customer dimension has a column for FirstPurchageDate I create a view upon Date dimension.
Kimball says: »Create the illusion of independent date dimensions by using views or aliases and uniquely label the columns.« It will be easy for an user in Power BI or Tableau to drag into a data model several views for fact and dimensions without thinking of a dimension is playing multiple roles. In a olap cube data model the fact can be joined multiple times to the same dimension and at Dimension Usage can each role be labeled, but since we are using the same dimension the column names will be reused.
Junk dimension, Garbage dimension, Abstract or Hybrid dimension
A single table with a combination of different and unrelated columns to avoid having a large number of foreign keys in the fact table and therefore have decreased the number of dimensions (dimensionality) in the fact table. Kimball recommand up to 25 dimensions in a fact. The content in the junk dimension table is the combination of all possible values of the individual columns called the cartesian product. For example, four different values that can be cross-joined into a Junk dimension:
Payment type: Cash or Credit card.
Coupon used: Yes or No or Not applicable.
Bag type: Fabric or Paper or Plastic or Unspecified.
Customer feedback: Good, Bad or None.
with 2 x 3 x 4 x 3 = 72 values or rows in a Junk dimension table, but can contain only the combination of values that actually occur in the source data.
The fact table only needs one key to the Junk dimension for getting the descriptive values of fact data for reporting. The pitfall of a Junk dimension is the filtering because a value (e.g. Credit card) exists as duplicate in multiple rows and therefore gives multiple key values to be joined to the fact table. To display unique content of a column from a Junk dimension in a dropdown or listbox I need to create a view for that column e.g. create view [Dim Bag type] as select distinct [Bag type] from [Dim Junk]. The view will handle a one-to-many relationship to the Junk dimension in the same way we handle a snowflake dimension. I can in Power BI create a calculated table upon the Junk dimension with a Dax like:
Dim Bag type = distinct(Dim Junk[Bag type]) and I build a relationship from the calculated table Dim Bag type back to the Junk dimension in the model where Junk dimension already has an one-to-many relationship back to the fact. I hide the Dim Junk because an user do not need it after we have calculated tables for each of the columns in the Junk dimension and therefore the Junk dimension become a bridge or a helper. I hope it shows how to use a Junk dimension in practice.
Degenerate dimension values exist in the fact table, but they are not foreign keys, and they do not join to a real dimension table. When the dimension value is stored as part of fact table, and is not in a separate dimension table, it is typically used for lowest grain or high cardinality dimensions such as voucher number, transaction number, order number, invoice number or ticket number. These are essentially dimension key for which there are no other columns, so a degenerate dimension is a dimension without columns or hierarchy. For example, the OrderNumber column can be in the Order fact table with several rows using the same order number, because one order can contain several products. Therefore the OrderNumber column is important to group together all the products in one order. Later for searching for an order number in a OLAP cube, a Order number dimension is very usefull, but it is not a dimension table, it is generated by the Order fact table and there is no additional data like a name or text.
Degenerate dimension is also used as an implemented Bridge dimension table in making a Multivalued dimension. In a data mart these are often used as the result of a drill through query to analyze the source of an aggregated number in a report. You can use these values to trace back to transactions in the OLTP system.
Normally a degenerate dimension is not a table in a database, it is a view with distinct values based on the fact table.
Dimensionalised dimension is a replacement of Degenerate dimension where the view become a materialized view meaning it become a table in the data mart and where the text column will be transformed to an integer key, like this:
SELECT QuantityPerUnit_key = CONVERT(BIGINT, HASHBYTES('MD5', t.QuantityPerUnit)),
FROM (SELECT DISTINCT QuantityPerUnit
FROM Products WHERE QuantityPerUnit IS NOT NULL) t
Static dimension or Constant dimension
Static dimensions are not extracted from the source legacy system, but are created within the context of the data warehouse or data mart. A static dimension can be loaded manually with Status codes or it can be generated by a procedure such as a Date dimension and Time dimension. The opposite would be called Dynamic dimension.
Several different kinds of entry with different columns for each fact (like sub classes). For example, heterogeneous products have separate unique columns and it is therefore not possible to make a single product table to handle these heterogeneous products.
Used to identify different facts that is populated in the same measure column in a fact table because the fact rows represent different entry typies. An entry type fact dimension describes what the fact row represents and how measures must be understand and used. The alternative is to have multiple measure columns for each entry type in the fact table where only one column has a value for each row.
Time machine dimension
A combination of two dimensions for Entry date (or event date) and for Year and where fact data rows is based on either »counterpart« as in a financial accounting or »transactions« as in a financial transactions. Section 6.4 will show an example.
A very large dimension that has a huge number of rows or many columns. For a real estate agent, I implemented a 132-columns dimension through a merge of five source legacy systems. The column names was made together with the users. The created dimension table got the column names in an alphabetical order so it is easy to find a specific column.
Dimensional columns are allowed to become complex objects rather than simple text like unstructured text, gps tracking, graphic images and in time series and in NoSQL bases like Hadoop and MongoDB, and become much more malleable and extensible from one analysis to another. Extensible design in software engineering is to accept that not everything can be designed in advance and extensibility is a software design principle defined as a system’s ability to have new functionality extended.
Audit dimension or Data quality dimension
A description of each fact table row would be »Normal value«, »Out-of-bounds value«, »Unlikely value«, »Verified value«, »Unverified value« and »Uncertain value«. Kimball recommands to load all rows to the fact table and use an Audit dimension to do a tagging of data because of an error condition and thereby to tell the state of each row in an audit report so an user can look at the data and make a data change or a counterpart in source legacy system and do a new load to the data warehouse to fix the data in the fact table. Data quality is monitored during the ETL process and it can procedure an audit statistics.
In a SSIS package a derived column can have an expression to validate data and give value to an Audit dimension where an amount from the source is less than 0 or more than 5000 gets audit key value 12 for out of bounds amount else value 0 for okay: Amount < 0 || Amount > 5000 ? (DT_I4)12 : (DT_I4)0
All rows are checked for compliance with the constraints.
An audit dimension can also have data for name and version of source legacy system, name of data table in source, time of extract from source, time of insert into the fact table etc.
4.5. Inferred members or Inferred dimensions
A dimension has a member value of »Unknown«, »Missing«, »N/A«, »-« etc. to handle source data that is going to be loaded into a fact table:
Approach: Handling a null/empty business key as a missing member because a row in the fact table for column of business key is not registered.
Approach: »Early arriving fact« for handling of orphaned data where the fact data has an unknown member, meaning the fact value is an orphan child because there is no parent value in the corresponding dimension table, the fact value does not exist in the dimension.
In a relational database it is called a referential integrity constraint violation in a table when a foreign key contains a value that does not exists as a primary key in a different (or the same) table. There is a no referred member and there is a need to have an inferred member. A forthcoming fact row has a member that will infer a new dimension member, therefore it is called inferred member of the dimension (in danish udledt), and it is to improve data quality and audit trail/control track for reconciliation in the data warehouse.
Handling a null/empty business key as a missing member
When a dataset is transformed to a fact table, there can be a business key column which value is empty or null, meaning it does not yet exist for the fact data. To keep all the fact data rows in the fact table, the related dimension table is already born with a »Missing« value (an inferred member) with surrogate key identity value -1, which can be used as a default value in the fact table foreign key column to the dimension table primary key column. Later the fact table with a -1 value can be updated with a real business key value that either exists in dimension table or will be inserted first and get a new surrogate key identity that the fact data row can refer to. Sometimes I have seen a dimension with an inferred member value of -1 for »Unknown«, but I prefer using -1 for »Missing« and using »Unknown« to handle the situation I will describe below, because I divide inferred members in two situations as the points shown above.
Early arriving fact as an unknown member
When a dataset is transformed to a fact table, there can be a business key column which value has not yet been received to the related dimension table and therefore does not yet exist. To keep all the fact data rows in the fact table, the related dimension table will first have to insert a new business key value with an »Unknown« value (an inferred member) which later will be updated in the dimension with the correct text value then it is known. The »Unknown« value gets the next surrogate key identity as an unique sequence number and will be used in fact table like any other dimension member value. A dimension table can at the same time have several »Unknown« member values with their own business key, surrogate key identity and the text value can include the value of the business key like »Unknown 886«, »Unknown 887«, »Unknown 888« and so on to distinct them for the users of the dimension. When a regular dimension has a hierarchy these levels can have text value »Unknown« as a special branch in the hierarchy.
Late arriving dimension
When a dimension data has been delayed:
a) If business key not exists then insert a new row.
b) If business key exists as an inferred member »Unknown« then type 1 update the row and type 2 insert a new row to keep history.
c) If business key exists and the delayed data comes with a date of valid then type 2 becomes more complex because validfrom and validto has to be adjusted and fact rows has to be revisited to update the dimension key column to point at the right dimension value at that date. You have to consider if it is allowed to change old data and old reporting result.
Late arriving fact
When a fact data has been delayed maybe it is including a date that can be used to search and fetch the current dimension member at that time if dimension keeps history. You have to consider if it is allowed to add a late arriving fact row because it will change an old report. For example, a boss already got the report of sales for the second quarter and at June 5 a late sale fact for March 31 is arriving and when it is added to the Sales fact the report for second quarter will change so it do not match the old reporting result.
When building the ETL process for a fact that is using type 2 or 7 dimensions sometimes we can assume that new fact data rows is at current date and therefore we only need to do a lookup for the dimension key value with this criteria:
dim.Businesskey = fact.Businesskey AND dim.IsCurrent = 1
But if a fact data row can be late arriving with an old date stamp in column RegisteredDate we need to do a range lookup for the dimension key value with a criteria to found the fact business key in the dimension at the time when fact date was valid in the dimension:
dim.Businesskey = fact.Businesskey AND
dim.EffectiveDate <= fact.RegisteredDate AND dim.ExpirationDate > fact.RegisteredDate
Range lookup is an in-memory lookup to assign a surrogate key to each coming fact row in a streaming ETL process of rows from a source legacy system to translate and replace a business key value with a dimension surrogate key value to be saved into a fact data row in a fact table.
Early arriving dimension
When a dimension has members that is not yet been referred to from a fact row or maybe it will never be i.e. a type of payment called »Blood«, but still keep it as a member of the dimension.
Other inferred members
Variety of constant inferred members would be like:
»Missing« (-1) when a source legacy system provide a business key value that is null, meaning it is not present in some data, not reported or »hasn’t happened yet« or »be available later« because the business key to be determined is expected to be available later and fact table will be updated thereby. Therefore the fact table column for the dimension gets the value -1 for missing.
»Not available« (-2) when a source legacy system provide a business key value that is not known in the data warehouse e.g. CustomerNumber 424-15-90. Therefore the fact table column for the dimension gets the value -2 for unknown in a general term for an unknown member because it is not all values I like to make as an inferred member for a dimension. A CustomerNumber in a source system could be type in wrongly and therefore is it not available for a dimension in the data warehouse, and I find it better to mark the dimension in the fact with -2 instead of inserting a wrong value into the Customer dimension. Some wrong CustomerNumbers will in the fact table gets the value -2 and therefore become homeless, but an Audit trail can fetch them and reporting them and the source system can be corrected.
»Not applicable« (-3) when the dimension is not relevant for the fact row (N/A, N.A.). It is best to avoid a N/A dimension by making separate fact tables.
»Bad«, »Corrupt« or »Dirty« (-4) when a source legacy system provide bad business key value or not enough data to determine the appropriate dimension key identity. This may be due to corrupted data in the source legacy system or incomplete knowledge of the business rules for the source data for the dimension.
A dimension table can have a metadata column like IsInferred as a boolean (bit) with two values (1 = true and 0 = false):
An example of a Customer dimension
Will an inferred dimension member value data row be updated when the source legacy system provide a Region and a Name?
The fact table is untouched or unchanged because the foreign key Customer_key values 134 - 136 are at the right place already, it is only Unknown values of the dimension that is changed. Kimball says there is no need for revisiting the fact table for making an inferred member to a normal member of the dimension.
When other levels in a hierarchy has text value »Unknown« like Region and users of the dimension in a cube has already made reports with fact data connected to »Unknown«, it is important to keep the history in the dimension so an user some months later still can make the same report with same summary value »Unknown«. If an user instead want to use the current values of the previous inferred members, the cube should provide such a dimension too.
The design of a dimensional model has a four-step process described by Kimball:
1. Select the business process to model by gathering and understanding business needs and available data in source legacy systems.
2. Declare the grain of fact as Transaction, Periodic or Accumulating/Aggregated.
3. Identify the dimensions with columns, hierarchy and group.
4. Identify the facts with measures as additive or non-additive.
Dimensional modeling ends up in relational database tables for dimensions and facts with one-to-many relationships between primary keys and foreign keys, meaning it become an Entity Relationship data model where entities are labeled with Kimball naming.
An entity or table can have a normal name e.g. Product and Sales and they can be placed into different database schemas based on Kimball naming to labeling each table in the dimensional modeling e.g. of some database schema names:
Extra schemas for Usage supporting tables in the ETL process:
Other data modeler is adding a label Dim and Fact to the name of the table e.g. DimProduct and FactSales or Product_dim and Sales_fact.
The most important of all is a good table name e.g. in Kimball’s Healthcare example he has tables like:
An invoice has multiple line items. Why make invoice as a dimension, when the parent invoice header fact table has 4 dimensions and 5 measures, and the child invoice line item fact table has 6 dimensions and 3 measures? Kimball tip 25: We can't roll up our business by product! If we constrain by a specific product, we don't know what to do with invoice level discounts, freight charges and tax. Have to take the invoice level data and allocate down to the line item level. Therefore we get one Invoice fact table with dimensions and measures from the invoice header e.g. invoice no, order no, invoice date, customer, ship-to and sales person, and from invoice line item e.g. product, quantity, unit price and amount.
Utilizing dimensional modeling, end users easily understand and navigate data structure and fully exploit the data.
5. Links for more readings
Ralph Kimball and Margy Ross: The Data Warehouse Toolkit, The Definitive Guide to Dimensional Modeling, Third Edition, 2013, first edition published in 1996.
Ralph Kimball and Joe Caserta: The Data Warehouse ETL Toolkit, Practical Techniques for Extracting, Cleaning, Conforming, and Delivering Data, 2004.
Page 215 shows a pipeline for a ETL process to lookup key values in dimensions, read how to implement a pipeline with range lookup for SSIS in section 11.4.
William H. Inmon: Building the Data Warehouse, Fourth Edition, 2005, first edition published in 1990.
6. Examples with sample values
6.1. Customer snowflake dimension
Customer table in a source legacy system with CustomerId as a primary key (called Business key in a data warehouse or data mart) and CustomerNumber as a secondary key including data of the nationality (country) and the gender of the customer:
Customer dimension table as a snowflake dimension:
Customer_key is an unique sequence number as primary key for the dimension for having independence on the business key CustomerId from the source legacy system. From CustomerNumber I derived two extra columns for Country and Gender and they become snowflake dimensions and can be used together with other dimension tables and fact tables. Payment is taking from another table in the CRM system and become a dimension itself and is merged into the Customer dimension to do an enrichment of the customers. Country dimension has been enriching inside the data warehouse by the business users with ISOcode and Currency values to fulfill reporting requirements. Each country is placed in a continent hierarchy (Africa, Antarctica, Asia, Australia, Europe, North America and South America) in case a report want a continent overview of the customers and sales.
Country dimension table becomes an Outrigger dimension:
Gender dimension table:
Payment dimension table with an extra data warehouse column to specify the sort order of payments for reporting purpose and a hierarchy of types:
For an olap multidimensional cube loading data to a Customer dimension will be implemented with a view that joins the relevant tables and with several columns and hierarchies to make an easy search, zoom and drill down to specific data in access data tools like Excel, Power BI, QlikView, Tableau or Targit:
· Nationality (Country renamed to play a role)
In terms of granularity I want to see daily sales by customer and product for each individual employee which gives the grain declaration of date, customer, product, employee. The time-of-day from the operational system is left out so quantity has been summarized to date level in case a customer purchase the same product at the same employee in the one day e.g. 10.15am and 3.45pm.
An example of a sales statistic would be: »What is the amount of sale to female customers from France that have been paid with credit card in April 2014?«
We need a Sales fact table like this:
The last row shows an unwanted sale because it is a stolen item therefore the customer is missing together with the amount.
A report want to show the total amount per gender including Transgender even though there is no customer yet. Since snowflaking has created a Gender dimension with all genders independent of customer, a query will use an outer join to include all genders:
SELECT g.Gender, TotalAmount = ISNULL(SUM(s.Amount),0)
FROM Fact.Sales s
INNER JOIN Dim.Customer c ON c.Customer_Key = s.Customer_key
RIGHT OUTER JOIN Dim.Gender g ON g.Gender_Key = c.Gender_key
GROUP BY g.Gender_key
When snowflaking is not wanted for the Customer dimension, it will contain all the relevant descriptive columns:
The Customer dimension will contain redundant data i.e. a PaymentSortorder value is repeated for all the customers that is using same payment. One day business ask for a new sort order of payments because it will be more suitable for the reporting. I recommend to have a Usage supporting database in the data warehouse with tables where business users can update data through an application. With a Payment table with columns Payment, PaymentSortorder and PaymentType a business user can change the values in column PaymentSortorder. The Usage supporting database will be a data source to the data warehouse where the ETL process will fetch the Payment table to the Input data area and the ETL process will update the Customer dimension table in the Data mart. Since business do not care of old sort order of payment, the column PaymentSortorder in Customer dimension will be treated as a type 1 column meaning a simple update of all the rows in the dimension table.
In the Usage supporting database a business user can add extra data to the countries and hereby enrich the reporting with data that doesn't exists in the source legacy systems.
Since there is no customer of Transgender, the value will not exists in the Customer dimension and therefore Transgender can’t been shown in a report except when it is added through an Union sql statement but data belongs in a row in a table.
When all dimension values is wanted as fact, the fact table needs to include extra dimensions for Gender, Country and Payment and drop them from Customer.
6.2. Type 1, type 2 and type 7 with most recent current value
A customer has Id 421 in a source legacy system and her maiden name is Marie Beaulieur, and she is inserted into a Customer dimension. At 2007-11-16 she got married and took her husband’s surname, therefore her name was changed to Marie Lesauvage.
When the dimension is a type 1, Marie’s maiden name will be overwritten and will be forgotten. Customer_key is a surrogate key identity column an unique sequence number.
The type 1 dimension will always display the customers most recent name (current or latest name). A Customer_key column will be in a fact table and in a data access tool like Tableau and QlikView fact column Customer_key will be joined to dimension column Customer_key inside the tool’s data model.
When the dimension is a type 2, Marie’s maiden name will remain and will be remembered and her changed name will continue in an extra row. To keep track of when a value is changed, there is two datetime columns called ValidFrom and ValidTo. Marie’s maiden name was valid from she was born or she become customer at 2000-05-08 and that date is known in the source legacy system. The first entering will be giving a ValidFrom e.g. 1900-01-01 as a beginning of time and a ValidTo e.g. 9999-12-31 as an end of time. Marie’s marriage at 2007-11-16 will create a new row in the dimension table with the ValidFrom 2007-11-16 because she took her husband’s surname and the previous row will get ValidTo 2007-11-16 meaning that until that date she kept her maiden name. Customer_key is a surrogate key identity column an unique sequence number.
Note: Kimball page 508: If ValidFrom and ValidTo are datetime stamps then the ValidTo must be set to exactly the ValidFrom of the next row so that no gap exists between rows and no overlap too with continuous time. Other kind of time is event time (in a point of time) and interval time with gaps or overlap.
ValidFrom represents »from now on« (in danish »fra og med«) and ValidTo represents »to and not included« (in danish »til og ikke medtaget«).
What happen to the date when Marie become a customer at 2000-05-08? We can add a column called CustomerBeginDate, or we can add two rows at the first time a customer is entering so ValidFrom in the second row will represent the date the person become a customer, it could look like this:
The above table is not used in the following example.
Marie Lesauvage remarried at 2014-07-15 and took her new husband’s surname therefore her name is changed to Marie Sainte and become a new row in the table.
The Business_key column is the »glue« that holds the multiple records together.
Columns ValidFrom and ValidTo is metadata information about the data row itself. A dimension can have extra columns for metadata for a data row in a table e.g.:
IsCurrent, IsDelete, IsInferred, State column or Status column with values e.g. Current, Deleted, Expired. More about metadata columns in section 6.3.
Marie has made six purchases which is entered into a fact table and the date of purchase determines the value in column Customer_key after this rule.
Business_key = CustomerId AND
ValidFrom <= PurchaseDate AND ValidTo > PurchaseDate
It is called Range Lookup when we are going to find the right Customer_key in the Customer dimension for a specific date of purchase for example, 2014-05-08 for Marie becomes:
Business_key = 421 AND
ValidFrom <= 2014-05-08 AND ValidTo > 2014-05-08
The dimension row with ValidFrom 2007-11-16 and ValidTo 2014-07-15 meets the rule:
Business_key = 421 AND
2007-11-16 <= 2014-05-08 AND 2014-07-15 > 2014-05-08
The dimension data row has Customer_Key value 4 which we are using in the inserted fact row that represent her purchase at 2014-05-08 where her surname was Lesauvage at that time. The Range Lookup will be performed for each row of purchase from the source legacy system where each PurchageDate must fit the rule. An example of a fact table where Marie has made many purchages since year 2005 and each purchage has its own Customer_Key value as a navigation reference back to the Customer dimension to fetch full name when the purchage was registered:
For each row in the fact table the value in column Customer_key is pointing to a row in the dimension table and give the name of the customer at the time the purchase occurred or was registered. Customer_key column in fact table represent as-was when it is joined to the dimension table.
When a report of purchases wants to show the customers most recent name (current or latest name) independent of the date of purchase, it is solved with a database view that find the most recent CustomerName for each value of Customer_key with this result:
A type 2 current view dimension can be implemented like this:
CREATE VIEW DimCustomer_Current AS
SELECT d.Customer_key, c.CustomerName
FROM DimCustomer d
(SELECT Business_key, CustomerName
WHERE ValidTo = '9999-12-31' -- datetime2(7) '9999-12-31 23:59:59.9999999'
) c ON c.Business_key = d.Business_key
The view will do a query for always to select the current value for each key values in the dimension and show the result set or recordset as showned in the table above.
If your sql has windowing function the view can avoid the self join which will enhance performance and here is no use of ValidTo column:
CREATE VIEW DimCustomer_Current AS
SELECT Customer_key, CustomerName = LAST_VALUE(CustomerName)
OVER(PARTITION BY Business_key ORDER BY ValidFrom
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)
Use the view in a join with the fact table to fetch the current value for each fact row:
SELECT c.CustomerName, f.Quantity, f.Amount
FROM FactSales f
INNER JOIN DimCustomer_Current c ON c.Customer_key = f.Customer_key
Customer_key column from the view represent as-is for most recent data when it is joined to the fact table.
The disadvance of the first view for current value of the dimension is the join on the business key because it can be a string representation or a composite business key is composed of multiple columns that needs to be part of the inner join columns, which can cost performance when the dimension has many rows. With type 7 dimension Kimball introducing an integer representation of the business key and he called it durable key, an artificial surrogate key value per business key, see later.
The disadvance of the second view is that the windowing function must be used for each column that is wanted to be displayed for the dimension.
Temporal tables in SQL Server 2016 is closest to type 4 because keeping history in the separate table while original dimension table keeps current dimension member, more reading.
In case you want to display the original value of the dimension as type 0:
CREATE VIEW DimCustomer_Original AS
SELECT Customer_key, CustomerName = FIRST_VALUE(CustomerName)
OVER(PARTITION BY Business_key ORDER BY ValidFrom
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)
Use the dimension table in a join with the fact table to fetch registered value for each fact row:
SELECT c.CustomerName, f.PurchaseDate, f.Quantity, f.Amount
FROM FactSales f
INNER JOIN DimCustomer c ON c.Customer_key = f.Customer_key
Customer_key column from the table of dimension represent as-was when the fact row was entered into the fact table when the data occurred.
There is another type 2 approach that do not need the ValidTo column in the dimension table and therefore the ETL process shall not update the previous row, because a ValidTo column can be calculated when it is needed to determine the dimension key for the coming fact row:
CREATE VIEW DimCustomer_ValidTo AS
SELECT Customer_key, CustomerName, ValidFrom,
ValidTo = LEAD(ValidFrom, 1, '9999-12-31')
OVER(PARTITION BY Business_key ORDER BY ValidFrom)
In a data access tool like Tableau and QlikView the type 2 for showing most recent values needs two views that will inside the tool’s data model be joined together. An user can in a dropdown box select Marie Sainte (meaning Customer_key value 5) and all her six purchases facts will be shown or a sum of Amount even though she has changed her name over time. The dimension for current customers will show this unique list of most recent customers in a dropdown box based on a view:
CREATE VIEW dma.DimCustomer_Current AS
SELECT Customer_key AS Customer_Current_key, CustomerName
WHERE ValidTo = '9999-12-31' -- datetime2(7) '9999-12-31 23:59:59.9999999'
The fact will show this through a view that include a Customer_Current_key column to be joined to the view DimCustomer_Current in the tool’s data model:
Column Customer_Current_key represents a master value for the different slave values in column Customer_key, e.g. master value 5 has slave values 3, 4, 5 in a master slave relationship.
CREATE VIEW dma.FactSales AS
WITH dimCustomer_Current (Customer_key, Customer_Current_key) AS
MAX(Customer_key) OVER(PARTITION BY Business_key) AS Customer_Current_key
SELECT f.PurchaseDate, f.Customer_key, c.Customer_Current_key, f.Product_key,
FROM FactSales f
INNER JOIN dimCustomer_Current c ON c.Customer_key = f.Customer_key
When current customer Marie Sainte is selected in a dropdown box, the tool will do a query based on the tool’s data model to show all her six purchases facts even though she has changed her name over time:
SELECT c.CustomerName, f.PurchaseDate, f.Quantity, f.Amount
FROM dma.FactSales f
INNER JOIN dma.DimCustomer_Current c
ON c.Customer_Current_key = f.Customer_Current_key
WHERE c.CustomerName = 'Marie Sainte'
The fact view is still keeping the Customer_key column to be joined to the Customer dimension for showing the name of a customer when the purchase was happening.
The view dma.FactSales is using a With part (Common Table Expression) to make a temporary result set. It could be changed to a materialized view which store data in a table with a clustered primary key through the ETL process, and table FactSales will join to the materialized-view-table to get a better performance.
When a dimension is a type 7 it keeps history as type 2 and it does the same as type 1 current value with an extra column called Durable key that is an integer representation of the business key because a business key could be a mix of number and text and could be a composite key.
Customer_key is holding the type 2 complete history of the dimension columns changes, and Customer_Durable_key is holding the type 1 current version of the dimension columns.
Both the dimension primary key (here Customer_key) and the durable key (here Customer_Durable_key) will appear in the fact table as a dual foreign key for a given dimension.
The Durable key will also appear in the current view that will show each customer in an unique list of most recent customers.
A type 7 current view dimension can be implemented like this:
CREATE VIEW DimCustomer_Current AS
SELECT Customer_Durable_key, CustomerName
WHERE ValidTo = '9999-12-31' -- datetime2(7) '9999-12-31 23:59:59.9999999'
The Durable key will also appear in a join with the fact table to fetch the current value for each fact row:
SELECT c.CustomerName, f.PurchaseDate, f.Quantity, f.Amount
FROM FactSales f
INNER JOIN DimCustomer_Current c
ON c.Customer_Durable_key = f.Customer_Durable_key
In a data access tool like Tableau and QlikView the type 7 current view become a dimension of most recent values and inside the tool’s data model the view is joined to the fact table’s durable key column. An user can in a dropdown box select Marie Sainte (meaning Customer_Durable_key value 3) and all her six purchases facts will be shown or a sum of Amount even though she has changed her name over time.
The advance with type 7 is that the business key from a source legacy system is not part of a query to fetch the most recent values from a dimension. See more about SCD type 7 dimension in next section.
6.3. Kimball type 7 dimension
A type 7 dimension will keep all values in a dimension independent of the source legacy system with an option to easy to roll up historical facts based on current dimension characteristics. It is formerly known as »Compliance-Enabled Data Warehouses« in Kimball Design Tip #74 that achieve:
· Eliminating Type 1 and Type 3 updates.
· Accessing all prior versions of a database at points in time.
· Protecting the custody of your data.
· Showing why and when changes to data occurred.
Durable key was presented in Design Tip #147 Durable “Super-Natural” Keys, and type 7 in Design Tip #152 Slowly Changing Dimension Types 0, 4, 5, 6 and 7.
Durable means there is no business rule that can change the key value because it belong to the data warehouse. A business key CustomerNumber with value 421 gets a durable key identifier with value 3, and it will be used in fact table to referring to the customer. When the CRM system will be replaced with a new system in the cloud, it is a task for the data warehouse to map the old CustomerNumber (421) to the new source legacy system CustomerId (836-Q) and thereby keep the durable key value 3 so the old fact rows will be unchanged. That makes the data warehouse resilient and robust.
Kimball says: »Slowly changing dimension type 7 is the ﬁnal hybrid technique used to support both as-was and as-is reporting. A fact table can be accessed through a dimension modeled both as a type 1 dimension showing only the most current values, or as a type 2 dimension showing correct contemporary historical proﬁles. The same dimension table enables both perspectives. Both the durable key and primary surrogate key of the dimension are placed in the fact table. For the type 1 perspective, the current ﬂag in the dimension is constrained to be current, and the fact table is joined via the durable key. For the type 2 perspective, the current ﬂag is not constrained, and the fact table is joined via the surrogate primary key. These two perspectives would be deployed as separate views to the BI applications.« link.
In this section, I like to give my own description of the dimension type 7 together with an example and how join data from fact to dimensions.
I will start with the life cycle terms for a value (member) in a dimension:
· Original is the value of birth when it entered and occurred first time.
· Historical is tracking the value for changes (modified/updated) at any time.
· Current is active, newest, latest of the value or most recent indicator.
· Deleted is the value no longer available in the source legacy system.
From a fact data point of view it is important to know the value of a dimension at the time of registration, therefore I will add to the life cycle:
· Registered is the dimension value when the fact data entered or occurred
(in danish registreret oprindelige værdi).
There will probably be dimension values that will not be used in fact data.
When I have one fact data row in my hand, I can provide four versions of the value in a dimension, can also be called four perspectives:
· Registered is the dimension value when the fact data row was registered or entered into the data warehouse with a transaction date or an insert time.
· Current is the latest and active dimension value and thus independent of the time of registration of the fact data row.
· Original is the entered first time dimension value maybe even before the fact data row exists in the source legacy system.
· Historical is the dimension value at a specific given date reporting point in time datetime independent of the fact transaction date.
Registered stands for as-was when occurred, Current stands for as-is for most recent data, Historical stands for as-of a specific particular reporting date or at any arbitrary date in the past and Original stands for as-began when the data value was opened, appeared or borned.
An additional information for the fact data row would be about the dimension value is not longer available in source legacy system because it has been marked expired or purged or it has been deleted.
My design pattern for type 7 dimension will have extra key columns or housekeeping columns or support columns or administrative columns:
· _key stands for a primary key column which is a new integer surrogate number every time a value in a source legacy system is changed and send to the data warehouse. The key is an unique identity auto increment sequential number normally starting from 1. Can be called _skey for surrogate key (SK) or even better _rkey for the registered key reference from a fact table row to the dimension table row with the textual data descriptive columns at the time the fact data row was registered or entered into the data warehouse.
· _dkey stands for a durable key which is not an unique integer number per row, instead it will hold the same number as the _key column when the dimension value entered the dimension table for the first time and that number will be repeated every time the value change. Can be called _ekey for entity key (EK) because the unique number of _dkey is also an unique independent data warehouse surrogate number for the business key which is unique in the source legacy system like a primary key. Sometimes called a stable key. Can also be called _hkey for history reference over time used for lookup in dimension for the current value, for the original value when it was entered the first time or for the historical value at a specific reporting datetime point in time.
· _pkey stands for a previous key (or parent-child key) which is a reference to a _key value for the previous or latest version of a dimension value when a value change over time. It is a referential integrity where _pkey is a foreign key to primary _key with one exception that value 0 in _pkey does not has a reference to a _key value because value 0 represent the first time the dimension value enter the dimension table and therefore has no previous value and that is the same as the original value.
· _bkey stands for a business key which is unique in the source legacy system like a primary key. A composite business key is composed of multiple columns like Name_bkey + Street_bkey + Zipcode_bkey and they will be multiple columns in the dimension table. Sometimes a string concatenation with a separator character to handle them in one _bkey column to compare with source data. Identical data can exists in multiple source legacy systems like an employee in CRM, ERP and HR with their own business key in the Employee dimension through sibling columns like CRM_bkey, ERP_bkey and HR_bkey with different representation like a number, a code and an email address. Depending of complexity there is a need of a mapping table in the data warehouse to take care of the merge of business keys and sometimes it is a manual task to merge data together when there is no suitable natural key. Importance is to have an employee as one entity with one durable surrogate number for the combination of business keys making a one-to-one relationship between _bkey and _dkey. Examples of concatenated columns with a checksum column called Comparison:
cast(hashbytes('sha2_256', concat(FirstName,';',LastName)) as binary(32))
Comparison_bkey AS (Convert([binary](16),hashbytes('MD5', concat([Name],';', Convert([varchar](23),[BirthDate],(126)))))) PERSISTED
used to find out when data has been changed in the source legacy system to make a new row in dimension with new validfrom/validto in the ETL process.
· _nkey stands for a natural key which is unique in the source legacy system like a candidate key or a secondary key and can be needed to conform a dimension across multiple source legacy systems which have their own business key that can’t be used in the merge because it is a surrogate key identity column an unique sequence number.
Lets have an example where a source legacy system has changed its value three times from »Warning« to »Notification« and to »Caution«. Each of the three values become a new row in a dimension table together with two metadata columns that representing the period for which the value was found in the source legacy system, here called:
· ValidFrom also called EffectiveDate or StartDateTime or BeginDateTime.
· ValidTo also called ExpirationDate or StopDateTime or EndDateTime.
A System dimension type 7 with key, metadata and descriptive columns:
_key stands for a primary key as a surrogate increment sequential number.
_dkey stands for a durable key is a integer representation of a business key.
_pkey stands for a previous key is reference to a _key value for the previous.
_bkey stands for a business key is the primary key in the source data.
Column _key is an unique primary key of the dimension table and the combination of _bkey + ValidFrom or _bkey + ValidTo is unique too.
[I prefer to think a table of a dimension as an object that has its own functions on data, therefore I like to have _key and _dkey to start from 1 for each dimension instead of having a dimension key store map with a global number for all the keys across all the dimensions where each business key will get its own surrogate key and a new dimension table will start with a _key like 4983562.]
»Warning« and »Notification« is history values of the System dimension and »Caution« is the most recent indicator of the System dimension with these terms:
· »Warning« is the original value.
· »Notification« is the historical value.
· »Caution« is the current value.
· »?« is the registered value depend of the transaction date in the fact row.
The challenge is that you have to choose which version of a dimension value you like to present in your report.
From a source legacy system the data warehouse has received two outages fact data with an OutageDate (kind of a transaction date) and a SystemId business key that match to System_bkey in the System dimension. Example of two source data rows:
For each source data row the value of OutageDate will look up in the System dimension at the time when the outage was happening to determine and fetch the correct dimension value. The fact data row is registered with criteria like:
System_bkey = SystemId AND
ValidFrom <= OutageDate AND ValidTo > OutageDate
It will select one row from the dimension table and the values of columns System_key and System_dkey will be copied to the new fact row, like for these two outages incidents at Maj 25 and August 2 in 2015 will become two fact data rows:
System_key is a reference to the System dimension value at the registered fact time from column OutageDate, and System_dkey is a reference to the entity with business key 76CB to provide current, original or historical value.
When we like to see the value from the dimension at the time where an outage happen, fact table column System_key will join to the dimension column System_key and get the value from the unique row. We call this value the registered value, because it was the value of the dimension when the outage incidents was occurred, and the value will never change because the dimension is handling all changes of values.
Fact table join to the dimension table to fetch registered value:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System d ON d.System_key = f.System_key
Here we see the purpose of the OutageDate and in case of the source legacy system does not provide a »transaction date« to determine and fetch the dimension value, the data warehouse will use a RegisteredTime (receipt datetime, createddate, inserttime, current_timestamp, getdate, sysdatetime) for when the fact data is registered or entered into the fact table row with criteria like:
dim.System_bkey = fact.SystemId AND
dim.ValidFrom <= fact.RegisteredTime AND dim.ValidTo > fact.RegisteredTime
It can sometimes confuse users because the value of System has changed over time, and what is the summation of ProductionKWh of the System when using the registered value? Therefore we can provide the current value of the System dimension.
Fact table join to the dimension table to fetch current value:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System d ON d.System_dkey = f.System_dkey AND
d.ValidTo = '9999-12-31 00:00:00'
And a summation of ProductionKWh is 394 KWh for Caution.
We can provide an original value of the System dimension.
Fact table join to the dimension table to fetch original value:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System d ON d.System_dkey = f.System_dkey AND
d.System_pkey = 0 (d.System_key = d.System_dkey)
And a summation of ProductionKWh is 394 KWh for Warming, which is the original value of the system, maybe some users remember it best.
We can provide a historical value of the System dimension at a specific given as-of date reporting point in time datetime (ReportTime, RequestedDate) that will be part of the criteria.
Fact table join to the dimension table to fetch historical value:
DECLARE @ReportTime datetime = '2015-06-01 10:24:47'
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System d ON d.System_dkey = f.System_dkey AND
d.ValidFrom <= @ReportTime AND d.ValidTo > @ReportTime
The »between-filter« do the constrained to a single version value of system. A reporting datetime at »June 1, 2015, 10.24« will fetch system Notification and that will never change even though the system name changes value over time. It means I can always repeat and reprocedure my report as long as I remember the reporting datetime. If I choose another reporting datetime like »September 1, 2015, 09.46« it will fetch system Caution. The reality with this approach is that you have a many-to-many relationship between the fact table and the dimension, so you must force your delivery reporting tools to choose an as-of date to allowing point in time analysis at a given date to avoid bringing back all combinations and resulting in overstated measurements.
It is hard to remember the criteria for _key, _dkey, ValidTo and ValidFrom therefore I will make four sql helper views to make the join between dimension and fact more easy for users and analysis tools:
CREATE VIEW dim.System_Current AS
SELECT System_dkey, System
WHERE ValidTo = '9999-12-31 00:00:00'
The view will do a query for always to select the current value for each values in the dimension.
Used in join with fact table to fetch current value for a fact row:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System_Current d ON d.System_dkey = f.System_dkey
Let us imagine we are back in time at June 5, 2015 and we like to see in Tableau the current system of the outage at May 25, the view System_Current will provide system Notification because that was the current value at that time. When we do the same in Tableau at August 22, the view System_Current will provide system Caution. Fact table column System_dkey will join to the view column System_dkey and it will provide different values from the dimension depend of the date and time we do the reporting in Tableau and the values will always be the current and most recent.
A dimension is used to search for data rows from a fact table and with the view System_Current it is easy inside Tableau and QlikView to make a dimension for recent values and join it to the fact through the column System_dkey in the tool’s data model. Tableau and QlikView shows the most recent values of System e.g. Caution and an user select the value in a dropdown as a criteria, and Tableau and QlikView do a join to the fact table and find all the data rows that is using this system even though the name of system has been changed over time:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System_Current d ON d.System_dkey = f.System_dkey
WHERE d.System = 'Caution'
In Tableau and QlikView the dimension would be named Current_System.
CREATE VIEW dim.System_Registered AS
SELECT System_key, System
Used in join with fact table to fetch registered value for a fact row:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System_Registered d ON d.System_key = f.System_key
In Tableau and QlikView the dimension would be named Registered_System and it is used to report a name of a system when the Outage was happening.
CREATE VIEW dim.System_Original AS
SELECT System_dkey, System
WHERE System_pkey = 0 (System_key = System_dkey)
Used in join with fact table to fetch original value for a fact row:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System_Original d ON d.System_dkey = f.System_dkey
Seldom to have an Original_System dimension in Tableau but we can if we want.
Historical function with a datetime parameter
CREATE FUNCTION dim.System_Historical(@ReportTime datetime)
RETURNS TABLE AS RETURN (
SELECT System_dkey, System
WHERE ValidFrom <= @ReportTime AND ValidTo > @ReportTime)
Used in join with fact table to fetch historical value for a fact row with a datetime:
SELECT d.System, f.OutageDate, f.ProductionKWh
FROM fact.Outage f
INNER JOIN dim.System_Historical('2015-06-01 10:24') d
ON d.System_dkey = f.System_dkey
Tableau and QlikView does not yet offer an user typein datetime value for a dimension before displaying the values of the dimension, but a report has normally some criteria boxes and one of them could be an input datetime for a historical dimension, so the report shows the values from a specific point in time, maybe to reproduce exactly the same report as it was made before at that time.
I prefer to make four database schemas: dimcurrent, dimhistorical, dimoriginal and dimregistered and reuse the name of the view System in all schemas, like I had multiple namespaces with their own objects. Then I write a sql statement I can in From/Inner Join part use IntelliSense e.g. »dimcurrent.« and I get the list of dimensions that presents current values that I want together with a fact table that I have placed in a schema called fact.
Dimension type 7 provides up to four values for registered, current, original and historical and it is very flexible for a fact data row to request for and fetch a dimension value. Of course a lot of fact data rows have only one value in a dimension, like a fact for sale to a new customer that is at the same time entered into the dimension. When the customer don’t change any data for a long time, the dimension value is registered = current = original = historical. The customer purchases many times and have many rows in sales fact, but one day the customer moves from his home country to three different countries over time and still continue to purchase. This means that the original value become the home country, historical values is the second and third country, and current value is the fourth and recent country, and the registered value country is depending of the date of the sale. For statistics of the sales we can have summaries for customers home countries even though they have moved to other countries because we simple use the original value of the dimension. We can have summaries for countries where the customer did their purchase from by using the registered value. In case we like to know the sales from customers who have once lived in a specific country, we first look up that country in the customer dimension and gets their _dkey’s and join them to the fact sale table. The marketing people like to know the current address, phone number and email of all the customers to contact them with new exciting offers. Type 7 gives you all the options and the four helper sql views is handy for ad hoc queries. The right information, at the right time, in the right place, in the right way, in the right format, to the right person. In a data access tool like Excel, Power BI, QlikView, Tableau or Targit it is easy to make dimensions based on the helper views and join the dimensions to the fact through the dkey or key columns in the tool’s data model.
Dimension table and Fact table
A dimension table in the physical design will have extra key columns or housekeeping columns or support columns or administrative columns:
· _key used for the registered fact data value of the dimension.
· _dkey used for the current, original and historical value of the dimension.
· _pkey used for the previous key that is a reference to the _key column.
· _bkey used for the business key from the source legacy system.
· _nkey used for the natural key from the source legacy system.
A fact table in the physical design has for each dimension two columns:
· _key used for the registered fact data value of the dimension.
· _dkey used for the current, original and historical value of the dimension.
With type 7 the fact table contains dual foreign keys for a given dimension; a surrogate key linked to the dimension table where type 2 attributes are tracked (_key and is primary key), and a durable supernatural key (also a surrogate key) linked to the dimension table where type 1 attributes are shown (_dkey and has duplicate values) through a view to display the current row in the type 2 dimension to present current attribute values.
A dimension table will keep track of datetime stamp of inserting and updating a row with two audit columns:
· InsertTime database server system time when the row was inserted.
· UpdateTime database server system time when the row was updated.
I don’t like an audit column to be Null or N/A, therefore UpdateTime gets the same datetime stamp as InsertTime, so both columns will have the same value until data in the row will be either changed or deleted in the source legacy system. The two audit columns can be assigned either by triggers for insert and update at the table or by the ETL process. I prefer the ETL process to handle all audit columns.
A dimension table will keep track of job, package or data flow execution identification stamp of inserting and updating a row with two audit columns:
· InsertExec execution identification when the row was inserted.
· UpdateExec execution identification when the row was updated.
Execution identification is usefull for an audit trail to do a log of number of rows that has been inserted or updated in a table in the data warehouse.
A dimension table will keep track of a current value in the source legacy system with a metadata column which is a boolean (bit) with two values (1 = true and 0 = false):
IsCurrent = 1 means that the row represent the current value from source system.
IsCurrent = 0 means that the row represent a historical value from source system.
Column IsCurrent can also be called NewestFlag, ActiveFlag or RecentFlag.
When IsCurrent = 1 the ValidFrom/ValidTo represent the period where the row value is current in the source legacy system.
A dimension table will keep track of an expired or deleted value in the source legacy system with a metadata column which is a boolean (bit) with two values (1 = true and 0 = false):
IsDeleted = 1 means that the row represent the deleted value from source system.
IsDeleted = 0 means that the row represent an existing value from source system.
Column IsDeleted can also be called DeleteFlag, DeletedAtSource, ExpiredFlag or InactiveFlag, when data are deleted in a system and therefore are gone forever.
When IsDeleted = 1 the ValidFrom/ValidTo represent the period where the row value does not exists in the source legacy system.
A dimension table can keep track of the reason why the row was changed in a column called RowChangeReason so it is easy for doing search and analysis. The column could have value »New« when the row represent a new customer, and later when customer move to another country the value could change to »Moved country« and later it would be »Changed credit status« and so on.
A fact table will keep track of datetime stamp of inserting a row with audit column:
· InsertTime database server system time when the row was registered.
When there is no transaction date for a fact data the InsertTime will be used to look up an unique dimension table row. Sometimes the term RegisteredTime is used.
A fact table will keep track of datetime stamp of updating a row with audit and metadata columns:
· UpdateTime database server system time when the row was updated.
· IsCurrent if fact row will change then keep track of current row.
· IsDeleted if fact row can be deleted from source legacy system.
· ValidFrom if fact row is treated as SCD type 2, Slowly Changing Facts.
· ValidTo if fact row is treated as SCD type 2, Slowly Changing Facts.
Slowly Changing Facts SCF or Frequently Changing Facts FCF but hopefully not Rapidly Changing Facts RCF because that could give a lot of facts rows over time.
When transactions are changed retroactively, you should update corresponding fact rows. If you need to track the history of fact rows it become Slowly Changing Facts, see more in section 6.4.
A developer like to prefix or suffix all key and audit and metadata columns to make sure that the source legacy system does not have a column with the same name. A suffix would be like _dw or _dwh or _audit or _meta.
Lets have an example where a source legacy system has changed its value three times from »Warning« to »Notification« and to »Caution« and in the ending the source legacy system will delete the value so it will not exists anymore, but the dimension will keep it forever. Each of the following steps will show how the dimension table looks like after each process has been done.
1. First time a value arrives to the data warehouse which happen at Maj 1, 2015, 5 pm or 17:00, a new row is added to the dimension table to contain the new value in column System together with key, audit, and metadata columns:
2. Value changed at 2015-05-08 therefore the first row is not current and a new row is added:
3. Value changed at 2015-07-31 therefore the second row is not current and a new row is added:
4. Value deleted at 2015-08-06 where the current row will be copied to a new row mark deleted:
With this approach there will be inserted an additional row for a deleted (or closed item) in the source legacy system, so the dimension will tell in the last row which is mark as deleted, a period where the value is deleted from 2015-08-06 10:10:05 to forever. IsDelete = 1 represent a deleted row or record or data in the source legacy system, therefore the business key is no longer available in source legacy system in the period of ValidFrom/ValidTo. No fact data with a _key column will never point to or refer to a deleted value in a dimension, therefore no fact row can use a _key value with IsDeleted = 1. IsCurrent = 0 represent the old historical version of the value and ValidFrom/ValidTo tells us the period when the old value was current or active back in time. Since older fact data can be referring by column _dkey to a deleted dimension value, it is still needed to have IsCurrent = 1, and with IsDeleted = 1 we can see that the value is not available in the source legacy system. Therefore a fact table data is consistent (not inconsistent) and will never be missing a dimension value in a dimension table. In case a deleted value will be reopen (reappear or reborn) with the same business key in the source legacy system, the current dimension row will be updated with IsCurrent = 0 and new ValidTo datetime, and a new row is inserted and become current, and we keep IsDeleted = 1 to show the value was deleted in a period.
When source data with an OutageDate arrives to the data warehouse it will find the historical system name at the time when the outage was happening with a criteria that will not include the deleted dimension value because a fact data with a _key column can’t refer to a deleted value:
IsDeleted = 0 AND System_bkey = SystemId AND
ValidFrom <= OutageDate AND ValidTo > OutageDate
Example of two source data rows:
Become two fact data rows where System_key is a reference to the System dimension value at the registered fact time from column OutageDate and System_dkey is a reference to the entity with business key 76CB to provide current, original or historical value.
The current view for the dimension to show all current values from the dimension will search after criteria IsCurrent = 1,