Unique

Definition

The UNIQUE constraint ensures that all values in a column (or combination of columns) are distinct across the table — no two rows can share the same value.

Example

CREATE TABLE users (
  email VARCHAR2(100) UNIQUE
);

Why Testers Verify This

A common test is attempting to insert a duplicate value (e.g., registering with an email that already exists) and confirming the application properly rejects it, both at the UI level and via a constraint violation at the database level.