Unique Constraint

A unique constraint ensures no two rows in a column (or set of columns) have the same value. Unlike a primary key, a table can have several unique constraints, and unique columns can still contain NULLs.

Multiple NULLs are allowed in a unique column because NULL represents an unknown value, and standard SQL never considers two unknowns "equal" to each other — see the glossary entry on NULL for the underlying comparison logic.

A common use case: a users table might have user_id as the primary key, but also a unique constraint on email, since email addresses need to be unique without being the table's primary identifier.

Example

ALTER TABLE employees
  ADD CONSTRAINT uq_employees_email UNIQUE (email);