NULL Predicate
Function
The NULL predicate tests a particular column to see if it contains the NULL value.
Invocation
One of the six predicates which constitute a search term.
Syntax
expression IS [NOT] NULL
where

expression | May reference any valid column even if it does not support NULL values.
NOT | Operator which negates the result of the predicate.

Description
This predicate tests to see if a given expression evaluates to a NULL value. The NULL predicate can only return either true (column IS NULL) or false (column holds a definite value). The result can never be unknown.
Example
The following example determines if any cruises were offered for which no reservations have been made:
SELECT ID_CRUISE
FROM CRUISE, CONTRACT
WHERE CRUISE_ID = ID_CRUISE AND DATE_RESERVATION IS NULL;