The UNIQUE constraint uniquely identifies each record in a database table. The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it. Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
Certain restrictions must be obeyed when working with unique and primary keys:
When the Table Editor is open, you can switch to the Constraints tab, and create (or edit) a key using the shortcut menu.
To delete a key, select a required key, and click Delete Constraint in the shortcut menu.