NULL Values and IS NULL

NULL means unknown or missing data - it's not zero, not empty string, it's the absence of a value.

You cannot use = to check for NULL. This won't work: WHERE email = NULL -- WRONG!

Instead, use IS NULL or IS NOT NULL:

SELECT * FROM employees WHERE manager_id IS NULL;

This finds employees with no manager: the people at the top of the company.

IS NOT NULL finds rows that DO have a value:

SELECT * FROM employees WHERE email IS NOT NULL;

Remember: NULL = NULL is not true in SQL!