The IS NULL Operator
Use the IS NULL operator to select rows that have no value in a particular column.
In Figure, we showed how all players with a league number can be found. This statement can also be formulated in another way, one that corresponds more to the original question.
43. Get the player number and the league number of each player who has a league number.
If NOT is left out, we get all the players who have no league number.
Figure. Get the name, the number, and the league number of each player whose league number is not equal to 8467.
A condition with IS NULL or IS NOT NULL can never have the value unknown; work this out by yourself.