Sometimes a dimension is deﬁned that has no content except for its primary key. For example, when an invoice has multiple line items, the line item fact rows inherit all the descriptive dimension foreign keys of the invoice, and the invoice is left with no unique content. But the invoice number remains a valid dimension key for fact tables at the line item level. This degenerate dimension is placed in the fact table with the explicit acknowledgment that there is no associated dimension table. Degenerate dimensions are most common with transaction and accumulating snapshot fact tables.