While working with databases, it's common to have empty or unknown "cells" in data tables. For example, in our first
flights table, we keep track of when flights arrive in a column called
In this table, the third flight was canceled and therefore did not have an arrive time. We'll leave this cell empty, which is denoted as
What do we do when we need to test whether a value is or is not null? We use the special keywords
IS NULL or
IS NOT NULL in the
WHERE clause (
= NULL does not work).