NOT NULL constraint in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the NOT NULL constraint do in a database?

It ensures that a column cannot have NULL values, meaning every row must have a value for that column.

beginner

How do you add a NOT NULL constraint to a column when creating a table?

Use column_name datatype NOT NULL in the CREATE TABLE statement.

intermediate

Can a column with a NOT NULL constraint accept empty strings ('')?

Yes, NOT NULL only prevents NULL values, but empty strings are considered valid values.

beginner

What happens if you try to insert a row without a value for a NOT NULL column?

The database will reject the insert and return an error because the column requires a value.

intermediate

How can you add a NOT NULL constraint to an existing column?

Use an ALTER TABLE statement like ALTER TABLE table_name ALTER COLUMN column_name SET NOT NULL; (syntax may vary by SQL dialect).

What does the NOT NULL constraint prevent?

A Deleting rows

B Inserting duplicate values

C Inserting NULL values into a column

D Updating values

The NOT NULL constraint stops NULL values from being inserted into the column.

Which SQL keyword is used to define a column that cannot be NULL?

A UNIQUE

B CHECK

C PRIMARY KEY

D NOT NULL

The NOT NULL keyword specifies that the column must have a value.

If a column is defined as NOT NULL, what happens if you try to insert a row without a value for that column?

A The insert fails with an error

B The database inserts a default value automatically

C The row is inserted with an empty string

D The row is inserted with NULL

The database rejects the insert because the column requires a non-NULL value.

Can a NOT NULL column contain an empty string ('')?

A Only if the column is numeric

B Yes, empty strings are allowed

C No, empty strings are treated as NULL

D Only if the column is a primary key

Empty strings are valid values and different from NULL, so they are allowed.

How do you add a NOT NULL constraint to an existing column in most SQL databases?

A ALTER TABLE table_name ALTER COLUMN column_name SET NOT NULL;

B ALTER TABLE table_name MODIFY column_name NOT NULL;

C ALTER TABLE table_name ADD NOT NULL column_name;

D ALTER TABLE table_name CHANGE column_name NOT NULL;

The standard way is to use ALTER COLUMN column_name SET NOT NULL, but syntax can vary by database.

Explain what the NOT NULL constraint does and why it is useful in a database.

Think about what happens if a column must always have a value.

Describe how you would add a NOT NULL constraint to a column in an existing table.

Consider how to change a column's rules after the table is created.

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