Slowly changing dimensions are a set of data warehouse techniques to deal with history within a dimension.
Dimensions are related to facts and while the facts are basically always transaction oriented with an associated date, a dimension is not. Therefore a dimension may require history as well.
3 basic types exist:
- Type 1, for which no history is kept,
- Type 2, for which history is kept for certain columns based on all changes at any time in a source system for individual records and
- Type 3, for which history is kept for a very limited set of columns based on changes at a certain point in time for a bulk set of records.