WHERE with IS NULL and IS NOT NULL in SQL - 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, meaning it has no data stored (NULL).

beginner

How do you select rows where a column is NOT empty in SQL?

Use WHERE column_name IS NOT NULL to find rows where the column has some value.

intermediate

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

Because NULL means unknown or missing data, and = compares values. Use IS NULL instead.

beginner

Write a SQL query to find all employees without a manager (manager_id is NULL).

SELECT * FROM employees WHERE manager_id IS NULL;

beginner

What is the difference between IS NULL and IS NOT NULL in a WHERE clause?

IS NULL finds rows where the column has no value. IS NOT NULL finds rows where the column has some value.

Which SQL clause correctly finds rows where the 'email' column has no value?

A WHERE email IS NOT NULL

B WHERE email = NULL

C WHERE email = ''

D WHERE email IS NULL

IS NULL checks for missing values. = NULL is invalid for this check.

What does WHERE phone_number IS NOT NULL do?

A Finds rows where phone_number is missing

B Finds rows where phone_number has a value

C Finds rows where phone_number equals zero

D Finds rows where phone_number is empty string

IS NOT NULL filters rows with any value present in the column.

Why is WHERE column = NULL not correct in SQL?

A Because NULL means unknown and can't be compared with =

B Because NULL is a reserved keyword

C Because = only works with numbers

D Because NULL is a string

NULL means unknown or missing, so = comparison doesn't work; use IS NULL.

Which query finds all records where 'birthdate' is known (not missing)?

A SELECT * FROM table WHERE birthdate IS NOT NULL

B SELECT * FROM table WHERE birthdate = NULL

C SELECT * FROM table WHERE birthdate IS NULL

D SELECT * FROM table WHERE birthdate = ''

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

If you want to find rows where a column 'score' is missing, which is correct?

A WHERE score = ''

B WHERE score = 0

C WHERE score IS NULL

D WHERE score IS NOT NULL

IS NULL finds missing or unknown values.

Explain how to use IS NULL and IS NOT NULL in a SQL WHERE clause and why they are important.

Think about how databases store missing information.

Write a simple SQL query to find all records where the 'address' field is missing.

Use IS NULL to check for missing data.