PostgreSQL Unique Constraint

last modified February 25, 2025

The UNIQUE constraint ensures that all values in a column are unique. This tutorial covers how to use the UNIQUE constraint with practical examples.

Setting Up the Database

First, let's create the users table with a UNIQUE constraint on the email column.

create_table.sql

CREATE TABLE users (
    user_id SERIAL PRIMARY KEY,
    username VARCHAR(50) NOT NULL,
    email VARCHAR(100) UNIQUE NOT NULL
);

The UNIQUE constraint ensures that no two users can have the same email address.

Basic Unique Constraint

This example demonstrates inserting unique emails into the users table.

unique_basic.sql

INSERT INTO users (username, email) VALUES
('john_doe', 'john@example.com'),
('jane_doe', 'jane@example.com');

Attempting to insert a duplicate email will result in an error.

Unique Constraint on Multiple Columns

This example creates a UNIQUE constraint on multiple columns.

unique_multiple_columns.sql

CREATE TABLE orders (
    order_id SERIAL PRIMARY KEY,
    user_id INT NOT NULL,
    product_id INT NOT NULL,
    UNIQUE (user_id, product_id)
);

The combination of user_id and product_id must be unique.

Adding Unique Constraint to Existing Table

This example adds a UNIQUE constraint to an existing table.

unique_add_to_existing.sql

ALTER TABLE users
ADD CONSTRAINT unique_username UNIQUE (username);

The username column must now contain unique values.

Dropping Unique Constraint

This example drops a UNIQUE constraint from a table.

unique_drop.sql

ALTER TABLE users
DROP CONSTRAINT unique_username;

The unique_username constraint is removed from the users table.

Unique Constraint with NULL Values

This example shows how UNIQUE constraints handle NULL values.

unique_with_null.sql

INSERT INTO users (username, email) VALUES
('alice', NULL),
('bob', NULL);

Multiple NULL values are allowed in a column with a UNIQUE constraint.

Unique Constraint with Index

This example creates a unique index on a column.

unique_index.sql

CREATE UNIQUE INDEX idx_unique_email
ON users (email);

The idx_unique_email index ensures that the email column contains unique values.

Best Practices for Using Unique Constraint

Use for Critical Data: Apply UNIQUE constraints to columns that must contain unique values.
Combine with NOT NULL: Use NOT NULL to ensure columns with UNIQUE constraints are always populated.
Index Performance: Unique constraints automatically create indexes, which can improve query performance.
Plan for Scalability: Consider the impact of unique constraints on large datasets and indexing strategies.

Source

PostgreSQL Documentation

In this article, we have explored how to use the PostgreSQL UNIQUE constraint with practical examples and best practices.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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