What happens if a constraint is violated?

The database rejects the operation that violates the constraint, preventing invalid data from being stored.

Can constraints be added after a table is created?

Yes, constraints can be added to existing tables using ALTER TABLE statements.

Are constraints enforced automatically by the database?

Yes, once defined, constraints are automatically enforced by the database engine during data modification operations.

Topic 1518 min readStructured Tutorial

Database Fundamentals: Constraints Overview

Quick Answer

Database constraints are rules applied to table columns to enforce data integrity and consistency. Common constraints include PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, and CHECK. They prevent invalid data entry and maintain relationships between tables.

Learning Objectives

Define what database constraints are and why they are important.
Identify and explain the main types of constraints in SQL.
Demonstrate how constraints enforce data integrity with examples.

Introduction

In relational databases, constraints are essential rules that ensure data remains accurate and reliable.

They help prevent invalid or inconsistent data from being entered into tables, which is critical for maintaining database integrity.

Data integrity is the foundation of reliable databases.

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Author

RPATechnology Team

Last Updated

June 2026

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Constraint	Purpose	Example
PRIMARY KEY	Uniquely identifies each record; cannot be NULL	id INT PRIMARY KEY
FOREIGN KEY	Ensures referential integrity between tables	user_id INT FOREIGN KEY REFERENCES users(id)
UNIQUE	Prevents duplicate values in a column	email VARCHAR(255) UNIQUE
NOT NULL	Disallows NULL values in a column	name VARCHAR(100) NOT NULL
CHECK	Enforces a condition on column values	age INT CHECK (age >= 18)

Learning Objectives

Introduction

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Recommended Next Topics

Relationships in Databases

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What Are Database Constraints?

Types of Constraints

Primary Key Constraint

Foreign Key Constraint

Unique and Not Null Constraints

Check Constraint

Examples of Constraints in SQL

Practical Example

Examples

Best Practices

Common Mistakes

Hands-on Exercise

Create a Table with Constraints

Interview Questions

What is the purpose of a primary key in a database?

How do foreign keys maintain data integrity?

Can a table have multiple primary keys?

MCQ Quiz

1. What is the role of a CHECK constraint in SQL?

2. Which of the following statements about UNIQUE constraints is true?

3. What is the role of a CHECK constraint in a SQL table?

4. Which of the following is TRUE about UNIQUE constraints?

Key Takeaways

Frequently Asked Questions

What happens if a constraint is violated?

Can constraints be added after a table is created?

Are constraints enforced automatically by the database?

Summary

SQL Server Management Studio (SSMS)

Creating a Database

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