What are Slowly Changing Dimensions?
Slowly Changing Dimensions (SCD) - dimensions that change slowly over time, rather than changing on regular schedule, time-base. In Data Warehouse there is a need to track changes in dimension attributes in order to report historical data. In other words, implementing one of the SCD types should enable users assigning proper dimension's attribute value for given date. Example of such dimensions could be: customer, geography, employee.
There are many approaches how to deal with SCD. The most popular are:
Type 0 - The passive method Type 1 - Overwriting the old value Type 2 - Creating a new additional record Type 3 - Adding a new column
Credit: http://datawarehouse4u.info/SCD-Slowly-Changing-Dimensions.htmlSlowly Changing Dimensions (SCD) - dimensions that change slowly over time, rather than changing on regular schedule, time-base. In Data Warehouse there is a need to track changes in dimension attributes in order to report historical data. In other words, implementing one of the SCD types should enable users assigning proper dimension's attribute value for given date. Example of such dimensions could be: customer, geography, employee.
There are many approaches how to deal with SCD. The most popular are:
In order to manage Slowly Changing Dimensions properly and easily it is highly recommended to use Surrogate Keys in the Data Warehouse tables.
A Surrogate Key is a technical key added to a fact table or a dimension table which is used instead of a business key (like product ID or customer ID).
Surrogate keys are always numeric and unique on a table level which makes it easy to distinguish and track values changed over time.
In practice, in big production Data Warehouse environments, mostly the Slowly Changing Dimensions Type 1, Type 2 and Type 3 are considered and used. It is a common practice to apply different SCD models to different dimension tables (or even columns in the same table) depending on the business reporting needs of a given type of data.
A Surrogate Key is a technical key added to a fact table or a dimension table which is used instead of a business key (like product ID or customer ID).
Surrogate keys are always numeric and unique on a table level which makes it easy to distinguish and track values changed over time.
In practice, in big production Data Warehouse environments, mostly the Slowly Changing Dimensions Type 1, Type 2 and Type 3 are considered and used. It is a common practice to apply different SCD models to different dimension tables (or even columns in the same table) depending on the business reporting needs of a given type of data.
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