Slowly changing dimensions (SCD)
“In general, dimensions change according to a certain pattern. Gradually values in the dimensions change, the so-called” Slowly Changing “of the dimensions. Ralph Kimball distinguishes a number of Slowly Changing Dimension type changes:
Type 1, where the old values are overwritten with the new values. An ordinary update on the existing row.
Type 2, adding a row to the table with the new values. The validity of each row is indicated by means of a start and an end date field. So here an update takes place on the old row to end its validity, followed by an insert of a row that reflects the new situation.
Type 3, which copies the old value to a history field within the same row, then overwrites the old value with the new value. The history is thus kept in the same row. Here only an update takes place on the existing row.