Uncommon
Slowly changing dimension
Structure in data warehousing
Slowly changing dimension
In data management and data warehousing, a slowly changing dimension (SCD) is a dimension that stores data which, while generally stable, may change over time, often in an unpredictable manner. This contrasts with a rapidly changing dimension, such as transactional parameters like customer ID, product ID, quantity, and price, which undergo frequent updates. Common examples of SCDs include geographical locations, customer details, or product attributes. Several techniques exist for handling slowly changing dimensions. Ralph Kimball's The Data Warehouse Toolkit popularized a numbered set of them, called Types 0 to 6. Type 0 treats an attribute as fixed. Type 1 overwrites the old value, so no history is kept. Type 2 adds a new row for each change, and Type 3 adds a column for the previous value. Type 4 keeps the history in a separate table. Types 5 and 6 are hybrids. Type 5 combines a Type 4 mini-dimension (a small separate table of related attributes) with a Type 1 pointer to the current...
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