Unique constraints

A unique constraint is a rule that the values of a key are valid only if they are unique in a table.

Unique constraints are optional and can be defined in the CREATE TABLE or ALTER TABLE statements with the PRIMARY KEY clause or the UNIQUE clause. The columns specified in a unique constraint must be defined as NOT NULL. A unique index enforces the uniqueness of the key during changes to the columns of the unique constraint.

A table can have an arbitrary number of unique constraints, with at most one unique constraint defined as a primary key. A table cannot have more than one unique constraint on the same set of columns.

A unique constraint that is referenced by the foreign key of a referential constraint is called the parent key.