Keys

A key is one or more expressions that are identified as such in the description of an index, unique constraint, or a referential constraint. The same expression can be part of more than one key.

A composite key is an ordered set of expressions of the same base table. The ordering of the expressions is not constrained by their ordering within the base table. The term value when used with respect to a composite key denotes a composite value. Thus, a rule such as the value of the foreign key must be equal to the value of the primary key means that each component of the value of the foreign key must be equal to the corresponding component of the value of the primary key.