Can UNIQUE constraints allow NULL values?

Yes, most database systems allow NULL values in UNIQUE columns, but the treatment of multiple NULLs can vary between systems.

What happens if I try to insert a duplicate value in a UNIQUE column?

The database will reject the insert or update operation and return an error indicating a UNIQUE constraint violation.

Is it possible to have more than one UNIQUE constraint in a table?

Yes, a table can have multiple UNIQUE constraints on different columns or sets of columns.

How does a UNIQUE constraint improve query performance?

UNIQUE constraints often create indexes on the constrained columns, which can speed up query operations involving those columns.

SQL UNIQUE Constraint - Complete Beginner Tutorial

RPATechnology.in

SQL UNIQUE Constraint - Complete Beginner Tutorial | RPATechnology.in

How to Use the UNIQUE Constraint

You can define a UNIQUE constraint when creating a table or add it later using ALTER TABLE.

It can be applied to a single column or a set of columns to enforce uniqueness across their combined values.

Syntax Examples for UNIQUE Constraint
Operation	SQL Syntax
Create table with UNIQUE column	CREATE TABLE Users (UserID INT, Email VARCHAR(255) UNIQUE);
Create table with composite UNIQUE constraint	CREATE TABLE Orders (OrderID INT, ProductID INT, UNIQUE (OrderID, ProductID));
Add UNIQUE constraint to existing table	ALTER TABLE Customers ADD CONSTRAINT unique_email UNIQUE (Email);

Examples

Example 1: UNIQUE Constraint on Single Column

CREATE TABLE Users (
  UserID INT PRIMARY KEY,
  Email VARCHAR(255) UNIQUE
);

INSERT INTO Users (UserID, Email) VALUES (1, 'user@example.com');
-- The following insert will fail due to duplicate Email
-- INSERT INTO Users (UserID, Email) VALUES (2, 'user@example.com');

This example creates a Users table where the Email column must be unique. Attempting to insert a duplicate email will result in an error.

Example 2: Composite UNIQUE Constraint

CREATE TABLE Orders (
  OrderID INT,
  ProductID INT,
  Quantity INT,
  UNIQUE (OrderID, ProductID)
);

INSERT INTO Orders (OrderID, ProductID, Quantity) VALUES (1, 101, 5);
-- The following insert will fail because the combination (1, 101) already exists
-- INSERT INTO Orders (OrderID, ProductID, Quantity) VALUES (1, 101, 10);

This example enforces uniqueness on the combination of OrderID and ProductID, preventing duplicate product entries for the same order.

Topic information

Learning Objectives

Introduction

Recommended Next Topics

NOT NULL Constraint

DEFAULT Constraint

CHECK Constraint

Constraint Management

Related Topics

Related Courses

C# Programming Course for Beginners to Advanced | Learn C# and .NET

What is the SQL UNIQUE Constraint?

How to Use the UNIQUE Constraint

UNIQUE vs PRIMARY KEY

Practical Examples

Example 1: UNIQUE on a Single Column

Example 2: Composite UNIQUE Constraint

Practical Example

Examples

Best Practices

Common Mistakes

Hands-on Exercise

Create a Table with UNIQUE Constraints

Add UNIQUE Constraint to Existing Table

Interview Questions

What is the purpose of the UNIQUE constraint in SQL?

Can a table have multiple UNIQUE constraints?

How does UNIQUE differ from PRIMARY KEY?

MCQ Quiz

Key Takeaways

Summary

Frequently Asked Questions

Can UNIQUE constraints allow NULL values?

What happens if I try to insert a duplicate value in a UNIQUE column?

Is it possible to have more than one UNIQUE constraint in a table?

How does a UNIQUE constraint improve query performance?

Best Practices

Introduction to Views

Java Programming Course for Developers and Interview Success | RPATechnology.in

Complete JavaScript Tutorial for Beginners to Advanced | JavaScript Programming Course

Complete MySQL Tutorial for Beginners to Advanced | MySQL Database Course

Python Programming Course for Beginners to Advanced | Learn Python Online

Complete React.js Tutorial for Beginners to Advanced | React Course