NOT NULL constraint in SQL

Introduction

The NOT NULL constraint makes sure a column always has a value. It stops empty or missing data in that column.

When you want to make sure every user has an email address in your database.

When storing product prices, and you want to avoid missing price values.

When recording dates of events that must always be known.

When you want to ensure a username is always provided during registration.

When you want to prevent saving incomplete records in any table.

Syntax

SQL

CREATE TABLE table_name (
  column_name data_type NOT NULL
);

You add NOT NULL right after the data type of the column.

This rule applies to each row, so every row must have a value in that column.

Examples

This creates a Users table where both id and name must have values.

SQL

CREATE TABLE Users (
  id INT NOT NULL,
  name VARCHAR(50) NOT NULL
);

Here, product_id and price cannot be empty for any product.

SQL

CREATE TABLE Products (
  product_id INT NOT NULL,
  price DECIMAL(10,2) NOT NULL
);

Every event must have an event_id and a date.

SQL

CREATE TABLE Events (
  event_id INT NOT NULL,
  event_date DATE NOT NULL
);

Sample Program

This example creates an Employees table where none of the columns can be empty. The first insert works fine. The second insert fails because email is NULL, which is not allowed. The SELECT shows the rows that were successfully added.

SQL

CREATE TABLE Employees (
  employee_id INT NOT NULL,
  first_name VARCHAR(30) NOT NULL,
  last_name VARCHAR(30) NOT NULL,
  email VARCHAR(50) NOT NULL
);

INSERT INTO Employees (employee_id, first_name, last_name, email) VALUES (1, 'Alice', 'Smith', '[e-mail]');

-- This will fail because email is NOT NULL
INSERT INTO Employees (employee_id, first_name, last_name, email) VALUES (2, 'Bob', 'Jones', NULL);

SELECT * FROM Employees;

Important Notes

NOT NULL helps keep your data complete and reliable.

If you try to insert NULL into a NOT NULL column, the database will give an error.

You can add NOT NULL when creating a table or alter an existing column to add it.

Summary

NOT NULL means a column must always have a value.

It prevents empty or missing data in important columns.

Use NOT NULL to keep your data clean and trustworthy.

Practice

1.

What does the NOT NULL constraint do in a database table?

easy

A. It automatically generates unique values for a column.

B. It allows a column to have duplicate values.

C. It ensures a column cannot have empty or missing values.

D. It deletes rows with null values.

Solution

1. Step 1: Understand the purpose of NOT NULL

   The NOT NULL constraint prevents a column from having NULL (empty) values, ensuring data is always present.

2. Step 2: Compare options with the definition

   Only It ensures a column cannot have empty or missing values. correctly describes this behavior; others describe different constraints or actions.

3. Final Answer:

   It ensures a column cannot have empty or missing values. -> Option C

4. Quick Check:

   NOT NULL means no empty values allowed [OK]

Hint: NOT NULL means no empty values allowed in a column [OK]

Common Mistakes:

• Confusing NOT NULL with UNIQUE constraint
• Thinking NOT NULL deletes rows
• Assuming NOT NULL auto-generates values

2.

Which of the following is the correct syntax to add a NOT NULL constraint to a column named email when creating a table?

CREATE TABLE users (
  id INT PRIMARY KEY,
  email ???
);

easy

A. NULL VARCHAR(255)

B. NOT NULL VARCHAR(255)

C. VARCHAR(255) NULL

D. VARCHAR(255) NOT NULL

Solution

1. Step 1: Recall correct order of column definition

   The data type comes first, then constraints like NOT NULL.

2. Step 2: Check each option's order

   VARCHAR(255) NOT NULL correctly places VARCHAR(255) before NOT NULL; others have wrong order or use NULL instead.

3. Final Answer:

   VARCHAR(255) NOT NULL -> Option D

4. Quick Check:

   Data type first, then NOT NULL [OK]

Hint: Data type comes before NOT NULL in column definition [OK]

Common Mistakes:

• Writing NOT NULL before data type
• Using NULL instead of NOT NULL
• Omitting data type

3.

Given the table employees with columns id INT PRIMARY KEY and name VARCHAR(100) NOT NULL, what happens when you run this query?

INSERT INTO employees (id, name) VALUES (1, NULL);

medium

A. The query fails with an error due to NOT NULL constraint.

B. The name column defaults to an empty string.

C. The row is inserted with a NULL name.

D. The row is inserted but name is ignored.

Solution

1. Step 1: Understand NOT NULL behavior on insert

   NOT NULL means the column cannot accept NULL values during insert or update.

2. Step 2: Analyze the insert statement

   The query tries to insert NULL into the name column, violating the NOT NULL constraint, causing an error.

3. Final Answer:

   The query fails with an error due to NOT NULL constraint. -> Option A

4. Quick Check:

   NOT NULL rejects NULL inserts [OK]

Hint: Inserting NULL into NOT NULL column causes error [OK]

Common Mistakes:

• Assuming NULL is converted to empty string
• Thinking the row inserts ignoring NULL
• Confusing NOT NULL with default values

4.

Consider this table creation statement:

CREATE TABLE products (
  product_id INT PRIMARY KEY,
  product_name VARCHAR(50) NOT NULL,
  price DECIMAL(5,2)
);

Which of the following statements will cause an error?

medium

A. INSERT INTO products (product_id, product_name, price) VALUES (1, 'Pen', 1.50);

B. INSERT INTO products (product_id, price) VALUES (2, 2.00);

C. INSERT INTO products (product_id, product_name) VALUES (3, 'Notebook');

D. INSERT INTO products (product_id, product_name, price) VALUES (4, 'Eraser', NULL);

Solution

1. Step 1: Identify columns with NOT NULL constraint

   Only product_name has NOT NULL, so it must have a value on insert.

2. Step 2: Check each insert statement

   INSERT INTO products (product_id, price) VALUES (2, 2.00); omits product_name, so it tries to insert NULL, causing an error. Others provide product_name.

3. Final Answer:

   INSERT missing NOT NULL column product_name causes error. -> Option B

4. Quick Check:

   NOT NULL columns must be included in insert [OK]

Hint: Always provide values for NOT NULL columns on insert [OK]

Common Mistakes:

• Ignoring NOT NULL columns in insert
• Assuming NULL allowed if column omitted
• Confusing NULL and empty string

5.

You have a table orders with columns order_id INT PRIMARY KEY, customer_id INT NOT NULL, and order_date DATE NOT NULL. You want to add a new column status that must always have a value and defaults to 'pending'. Which is the correct way to add this column?

hard

A. ALTER TABLE orders ADD COLUMN status VARCHAR(20) NOT NULL DEFAULT 'pending';

B. ALTER TABLE orders ADD COLUMN status VARCHAR(20) DEFAULT 'pending';

C. ALTER TABLE orders ADD COLUMN status VARCHAR(20) NOT NULL;

D. ALTER TABLE orders ADD COLUMN status VARCHAR(20);

Solution

1. Step 1: Understand NOT NULL with default value

   Adding a NOT NULL column requires a default value to avoid errors on existing rows.

2. Step 2: Analyze each ALTER statement

   ALTER TABLE orders ADD COLUMN status VARCHAR(20) NOT NULL DEFAULT 'pending'; adds status with NOT NULL and default 'pending', so existing rows get this value. Others miss NOT NULL or default.

3. Final Answer:

   ALTER with NOT NULL and DEFAULT 'pending' is correct. -> Option A

4. Quick Check:

   NOT NULL columns need default when added to existing table [OK]

Hint: Add NOT NULL column with DEFAULT to avoid errors [OK]

Common Mistakes:

• Adding NOT NULL column without default
• Assuming default alone enforces NOT NULL
• Omitting NOT NULL when required