Slowly Changing Dimension

Also known as: SCD

A dimension whose attribute values change occasionally over time, together with the technique chosen for whether to keep or overwrite the prior values.

In practice: Pick a strategy per attribute, not per table:

• Type 1 — overwrite the value; no history kept.
• Type 2 — add a new row with a new surrogate key and an effective-date range (valid_from / valid_to / is_current); full history preserved.
• Type 3 — keep a “previous value” column alongside the current one; limited history.

Example: A customer moves city. Type 1 overwrites the city; Type 2 closes the old row (valid_to) and inserts a new current row, so historical facts still join to the address that was true at the time.

See also: Kimball Keys Definitions · Standard Cost