# 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.

```sql
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](https://www.ask-codey.com/glossary/subjects/database/novice)

Updated: September 27, 2026

Source: https://www.ask-codey.com/glossary/unique-constraint