Degenerate Dimensions

Degenerate dimension is a term attributed to Ralph Kimball who defined it as:

A dimension key in the fact table that does not have its own dimension table, because all the interesting attributes have been placed in analytic dimensions.

Bob Becker, a member of the Kimball Group, explained the degenerate dimension further.

Degenerate dimensions commonly occur when the fact table's grain is a single transaction or transaction line. Transaction control header numbers assigned by the operational business process are typically degenerate dimensions, such as order numbers, ticket numbers, credit card transaction numbers, or check numbers. These degenerate dimensions are natural keys of the "parents" of the line items.

Even though there is no corresponding dimension table of attributes, degenerate dimensions can be useful for grouping together related fact table rows. For example, retail point-of-sale transaction numbers tie all the individual items purchased together into a single market basket. In health care, degenerate dimensions can group the claims items related to a single hospital stay or episode of care.

Using a degenerate dimension saves space and improves performance.