NULL represents a missing or unknown value. You can check for
NULL values using the expression
IS NULL. For example, to count the number of missing birth dates in the
WHERE birthdate IS NULL;
As you can see,
IS NULL is useful when combined with
WHERE to figure out what data you're missing.
Sometimes, you'll want to filter out missing values so you only get results which are not
NULL. To do this, you can use the
IS NOT NULL operator.
For example, this query gives the names of all people whose birth dates are not missing in the
WHERE birthdate IS NOT NULL;