It is a dimension which is derived from fact table. DD doesn’t have separate dimension table. It is mostly used when we planned use transactional level information on the fact table. The main purpose of DD is to provide the info directly without referring other table.
Primary key of a fact table is subset of table's foreign key. In some cases, we do not need the foreign key in the fact guarantee the uniqueness, in that scenario, DD plays the role.
Dimension keys without corresponding dimension tables is known as Degenerated Dimensions
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