Slowly Changing Dimensions (SCD)

A "Slowly Changing Dimension" is a dimension whose attribute values change occasionally over time - a customer moves to a new address, a product is renamed, a store is reassigned to a new region etc. The changes are infrequent and unpredictable, unlike facts which arrive constantly.

The "SCD Types" are the standard techniques for deciding what to do when a source value changes: ignore it, overwrite it or keep the old value alongside the new one.

SCD Types apply to attributes, not tables

Engineers often label an entire table as a "Type 2 dimension", but the type is actually chosen per column. For example, a single dim_customer table can freeze its sign-up date, overwrite its email and track the full history of its address. This applies three different techniques within the same table.