What happens if I try to insert NULL into a NOT NULL column?

The database will reject the insert or update operation and return an error.

Can NOT NULL columns have default values?

Yes, you can define default values for NOT NULL columns to automatically insert a value if none is provided.

Is NOT NULL the same as a Primary Key?

No, but Primary Keys implicitly have NOT NULL constraints because they must uniquely identify rows.

Topic 13118 min readStructured Tutorial

SQL NOT NULL Constraint - Complete Beginner Tutorial

Quick Answer

The SQL NOT NULL constraint ensures that a column cannot have NULL values, enforcing that every record must contain a valid value in that column. This is crucial for data integrity and avoiding missing information in databases.

Learning Objectives

Understand what the NOT NULL constraint does in SQL.
Learn how to apply the NOT NULL constraint to table columns.
Recognize the importance of NOT NULL for data integrity.

Introduction

In SQL, constraints are rules applied to table columns to enforce data integrity.

One of the most common constraints is NOT NULL, which ensures that a column cannot contain NULL values.

This tutorial explains the NOT NULL constraint, its purpose, and how to use it effectively.

Data integrity starts with ensuring required fields are never left empty.

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Author

RPATechnology Team

Last Updated

June 2026

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Operation	Syntax Example
Create Table with NOT NULL	CREATE TABLE Employees (ID INT NOT NULL, Name VARCHAR(100) NOT NULL);
Add NOT NULL to Existing Column	ALTER TABLE Employees MODIFY Name VARCHAR(100) NOT NULL;

Learning Objectives

Introduction

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Recommended Next Topics

UNIQUE Constraint

CHECK Constraint

DEFAULT Constraint

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What is the NOT NULL Constraint?

How to Use NOT NULL in SQL

Example: Creating a Table with NOT NULL Columns

SQL Code Example

Practical Example

Examples

Best Practices

Common Mistakes

Hands-on Exercise

Create a Table with NOT NULL Columns

Interview Questions

What does the NOT NULL constraint do in SQL?

Can you add a NOT NULL constraint to an existing column?

Why is NOT NULL important for database design?

MCQ Quiz

1. What is the primary purpose of the NOT NULL constraint in SQL?

2. How can you apply a NOT NULL constraint to an existing column in a table?

3. Consider the following SQL statement: CREATE TABLE Employees (ID INT NOT NULL, Name VARCHAR(100) NOT NULL, Department VARCHAR(50)); What does the NOT NULL constraint on ID and Name imply?

4. Which of the following is a common mistake when using the NOT NULL constraint?

5. Why is the NOT NULL constraint important for database design?

Key Takeaways

Frequently Asked Questions

What happens if I try to insert NULL into a NOT NULL column?

Can NOT NULL columns have default values?

Is NOT NULL the same as a Primary Key?

Summary

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