UNIQUE Constraint Makes sure that all values in a column are different.

Example: Email addresses must be unique.

CREATE TABLE users (
    user_id SERIAL PRIMARY KEY,
    email VARCHAR(100) UNIQUE
);

Why do we need UNIQUE?
In real life, some data should never repeat.
Example: Two users cannot have the same email address.
Example: Two cars cannot have the same registration number.

That’s where the UNIQUE constraint comes in.
Step 1: Create a Table with UNIQUE Constraint

CREATE TABLE users (
    user_id SERIAL PRIMARY KEY,
    username VARCHAR(50) UNIQUE,
    email VARCHAR(100) UNIQUE
);

Here: username must be unique. email must also be unique.

Step 2: Insert Valid Data

INSERT INTO users (username, email) VALUES ('ravi', 'ravi@example.com');
INSERT INTO users (username, email) VALUES ('kavya', 'kavya@example.com');

Both insertions succeed.

Step 3: Try Inserting Duplicate Value

INSERT INTO users (username, email) VALUES ('ravi', 'ravi123@example.com');

ERROR: duplicate key value violates unique constraint "users_username_key"
DETAIL: Key (username)=(ravi) already exists.

PostgreSQL stops us from inserting duplicate usernames.