IS NULL and IS NOT NULL in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SQL condition IS NULL check for?

It checks if a column's value is missing or unknown, meaning it has no value stored (NULL).

beginner

How do you find rows where a column has any value (not NULL)?

Use IS NOT NULL to select rows where the column has a value stored (not missing).

intermediate

Why can't you use = NULL to check for NULL values?

Because NULL means unknown, and comparing with = doesn't work. You must use IS NULL or IS NOT NULL.

beginner

Write a simple SQL query to find all employees without a phone number (phone_number is NULL).

SELECT * FROM employees WHERE phone_number IS NULL;

beginner

What is the difference between IS NULL and IS NOT NULL?

IS NULL finds rows where the value is missing (NULL). IS NOT NULL finds rows where the value exists (not NULL).

Which SQL condition correctly finds rows where a column is missing a value?

A IS NOT NULL

B = NULL

C IS NULL

D <> NULL

Use IS NULL to check for missing (NULL) values. = NULL does not work.

What does IS NOT NULL do in a SQL query?

A Finds rows where the column is missing

B Finds rows where the column has a value

C Deletes rows with NULL

D Updates NULL values

IS NOT NULL selects rows where the column has any value (not NULL).

Why can't you use WHERE column = NULL to find NULL values?

A Because NULL is a string

B Because = NULL is the same as IS NULL

C Because NULL is zero

D Because NULL means unknown and = NULL always returns false

NULL means unknown, so = NULL does not work. Use IS NULL instead.

Which query finds all customers with a known email address?

A SELECT * FROM customers WHERE email IS NOT NULL;

B SELECT * FROM customers WHERE email IS NULL;

C SELECT * FROM customers WHERE email = NULL;

D SELECT * FROM customers WHERE email <> NULL;

IS NOT NULL finds rows where email has a value.

If you want to find rows where a column is not missing, which condition do you use?

A IS NOT NULL

B IS NULL

C = NULL

D != NULL

IS NOT NULL finds rows where the column has a value.

Explain how to check for missing values in a SQL column and why you use that method.

Think about how SQL treats unknown values.

Describe the difference between IS NULL and IS NOT NULL in SQL queries.

One finds missing data, the other finds present data.

Practice

1. What does the SQL condition column_name IS NULL check for in a table?

easy

A. It checks if the column value is missing or empty.

B. It checks if the column value is zero.

C. It checks if the column value is a blank string.

D. It checks if the column value is greater than zero.

Solution

1. Step 1: Understand the meaning of NULL in SQL

   NULL means the value is missing or unknown, not zero or empty string.

2. Step 2: Analyze the condition IS NULL

   This condition returns true only when the column has no value (NULL).

3. Final Answer:

   It checks if the column value is missing or unknown. -> Option A

4. Quick Check:

   IS NULL means missing value check [OK]

Hint: IS NULL means value is missing, not zero or empty [OK]

Common Mistakes:

• Confusing NULL with zero or empty string
• Using = NULL instead of IS NULL
• Thinking NULL means zero

2. Which of the following is the correct syntax to select rows where the column email is NOT NULL?

easy

A. SELECT * FROM users WHERE email IS NOT NULL;

B. SELECT * FROM users WHERE email = NOT NULL;

C. SELECT * FROM users WHERE email != NULL;

D. SELECT * FROM users WHERE NOT email = NULL;

Solution

1. Step 1: Recall correct syntax for NOT NULL check

   In SQL, to check for NOT NULL, use IS NOT NULL.

2. Step 2: Evaluate each option

   SELECT * FROM users WHERE email IS NOT NULL; uses correct syntax. Others use invalid or incorrect operators with NULL.

3. Final Answer:

   SELECT * FROM users WHERE email IS NOT NULL; -> Option A

4. Quick Check:

   Use IS NOT NULL to check non-missing values [OK]

Hint: Use IS NOT NULL, not = or != with NULL [OK]

Common Mistakes:

• Using = NULL or != NULL instead of IS NULL/IS NOT NULL
• Forgetting IS keyword
• Using NOT email = NULL which is invalid

3. Given the table orders with columns order_id and shipped_date, what will this query return?

SELECT order_id FROM orders WHERE shipped_date IS NULL;

medium

A. All orders that have a shipped date recorded.

B. All orders that have not been shipped yet.

C. All orders with shipped_date equal to zero.

D. Syntax error due to IS NULL usage.

Solution

1. Step 1: Understand what IS NULL filters

   The condition shipped_date IS NULL selects rows where shipped_date is missing.

2. Step 2: Interpret the meaning in context

   Missing shipped_date means the order has not been shipped yet.

3. Final Answer:

   All orders that have not been shipped yet. -> Option B

4. Quick Check:

   IS NULL finds missing shipped dates = unshipped orders [OK]

Hint: IS NULL finds missing data, here unshipped orders [OK]

Common Mistakes:

• Thinking IS NULL returns shipped orders
• Confusing NULL with zero or empty string
• Assuming syntax error with IS NULL

4. You wrote this query but it returns no rows:

SELECT * FROM employees WHERE manager_id = NULL;

What is the error and how to fix it?

medium

A. Add quotes around NULL like = 'NULL'.

B. Change manager_id to IS NOT NULL.

C. Use IS NULL instead of = NULL to check for NULL values.

D. Use != NULL to find rows with NULL.

Solution

1. Step 1: Identify incorrect NULL comparison

   Using = NULL always returns false because NULL is not equal to anything.

2. Step 2: Correct syntax to check NULL

   Use IS NULL to check for NULL values properly.

3. Final Answer:

   Use IS NULL instead of = NULL to check for NULL values. -> Option C

4. Quick Check:

   Use IS NULL, not = NULL, for NULL checks [OK]

Hint: Always use IS NULL, never = NULL for missing values [OK]

Common Mistakes:

• Using = NULL or != NULL instead of IS NULL/IS NOT NULL
• Adding quotes around NULL
• Confusing IS NULL with IS NOT NULL

5. You want to find customers who have provided a phone number but no email. Which query correctly uses IS NULL and IS NOT NULL to do this?

SELECT * FROM customers WHERE phone IS NOT NULL AND email IS NULL;

hard

A. This query has syntax errors with IS NULL usage.

B. This query returns customers missing both phone and email.

C. This query returns customers with both phone and email.

D. This query returns customers with phone and missing email.

Solution

1. Step 1: Analyze the conditions

   phone IS NOT NULL selects customers who provided a phone number.

2. Step 2: Check the email condition

   email IS NULL selects customers who did not provide an email.

3. Step 3: Combine conditions with AND

   Both conditions together find customers with phone but no email.

4. Final Answer:

   This query returns customers with phone and missing email. -> Option D

5. Quick Check:

   IS NOT NULL and IS NULL combined filter correctly [OK]

Hint: Combine IS NOT NULL and IS NULL with AND for mixed filters [OK]

Common Mistakes:

• Mixing AND with OR incorrectly
• Using = NULL instead of IS NULL
• Confusing which column to check for NULL