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.