Composite key

In database design, a compound key (also called a composite key or concatenated key) is a key that consists of 2 or more attributes.

No restriction is applied to the attributes regarding their (initial) ownership within the data model. This means that any one, none, or all, of the multiple attributes within the compound key can be foreign keys. Indeed, a foreign key may, itself, be a compound key.

Compound keys almost always originate from attributive or associative entities (tables) within the model, but this is not an absolute.