UNIQUE Constraint

A constraint preventing duplicate values or value combinations in specified columns.

What does UNIQUE Constraint do in a relational database?

UNIQUE Constraint — A constraint preventing duplicate values or value combinations in specified columns.

CREATE TABLE customer (
    customer_id INTEGER PRIMARY KEY,
    email CHARACTER VARYING(255) NOT NULL,
    status CHARACTER VARYING(20) DEFAULT 'active'
);

Level: Novice · More Database terms at this level

Updated September 27, 2026