Slowly changing dimension

In data warehousing, a slowly changing dimension (SCD) is the problem of representing historical changes to master data — customers, products, locations — without destroying the historical record. Type 1 SCDs overwrite old data; Type 2 SCDs create new rows with validity dates; Type 3 SCDs add columns to track previous values. The choice between types is not merely technical but epistemological: it encodes an organization's theory of what constitutes identity across time. A Type 2 SCD treats the customer as a temporal entity with a history; a Type 1 SCD treats the customer as a state to be updated. The Data warehouse is not just a database of facts; it is a database of ontologies, and the SCD is where those ontologies are made explicit. The organizations that default to Type 1 are the ones that have not yet decided whether they care about history.