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.
The following SQL creates a UNIQUE constraint on the "c_id" column when the "customer" table is created:
CREATE TABLE customer ( c_id INT NOT NULL UNIQUE, lastName VARCHAR(255) NOT NULL, firstName VARCHAR(255), address VARCHAR(255), )