Slowly Changing Dimension (SCD)

A slowly changing dimension (SCD) is any dimension whose descriptive attributes change over time — customers move cities, products get recategorised — forcing a choice about history. The SCD types name the standard answers.

Type 1 overwrites: keep only the current value, lose history — last year’s sales appear under the customer’s current city. Type 2 versions: insert a new row for each change, with validity dates and its own surrogate key, so facts join to the version true at the time. Type 2 keeps history honest at the cost of more rows and careful ETL. The right type per attribute is a business decision disguised as a technical one: do old orders belong to the old segment, or the new?

Go deeper: Slowly Changing Dimensions, Explained Without the Jargon