You can use UNIQUE constraints to ensure that no duplicate values are entered in specific columns that do not participate in a primary key. Although both a UNIQUE constraint and a PRIMARY KEY constraint enforce uniqueness, use a UNIQUE constraint instead of a PRIMARY KEY constraint when you want to enforce the uniqueness of a column, or combination of columns, that is not the primary key.
Multiple UNIQUE constraints can be defined on a table, whereas only one PRIMARY KEY constraint can be defined on a table.
Also, unlike PRIMARY KEY constraints, UNIQUE constraints allow the value NULL. However, as with any value participating in a UNIQUE constraint, only one NULL value is allowed per column.
A UNIQUE constraint can also be referenced by a FOREIGN KEY constraint.