Determine support requirements for slowly changing dimensions

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Because of LOB application limitation, the requirement of keeping track of changes of dimension values over time for historical reporting is commonly delegated to the data warehouse and is known as the Slowly Changing Dimension (SCD) concept.

There are several ways of handling changes to dimension values. The three most common
ways are known as Type 0, Type 1, and Type 2 SCD.

Type 0 SCD
Ignores changes to attribute values. In other words, the original dimension values that were initially inserted persist regardless of changes over time.

Commonly used for attributes that you do not expect to change, for example, a customer’s date of birth or Social Security number.

Type 1 SCD
Tracks changes by overwriting the current attribute value. This type of SCD simply replicates the change from the source system in the data warehouse. 

Type 1 SCD is commonly used for those attributes that are not relevant for historical reporting, such as a customer’s phone number.

Type 2 SCD
Tracks changes by inserting a new record with the changed values and keeping the old record with the original attribute values. 

This type of SCD typically requires two metadata columns to store the start and end dates that the record is valid for. These dates are used as lookup values during fact table loads to determine the corresponding dimension member key that was valid at the time the transaction took place.


Keep in mind that you can define all attributes to be handled the same way or you can define certain attributes to be handled as one type of SCD and other attributes as a different type of SCD.

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