The NULL predicate tests for null values.
The result of a NULL predicate cannot be unknown. If the value of the expression is null, the result is true. If the value is not null, the result is false.
If NOT is specified, the result is reversed.
Syntax alternatives: For compatibility with other SQL dialects, you can use ISNULL as a synonym for IS NULL and NOTNULL as a synonym for IS NOT NULL.
EMPLOYEE.PHONE IS NULL SALARY IS NOT NULL