Testing for an Unknown Value

Another type of comparison tests for the absence or presence of a value. Use the IS NULL operator to test whether a value is unknown. To test for the presence of any value, use IS NOT NULL.

Testing for NULL

1. Execute the following query to retrieve the names of employees who do not have phone extensions:

SELECT last_name, first_name, phone_ext FROM Employee=
WHERE phone_ext IS NULL

The query should return rows for last names Sutherland, Glon, and Osborne.

2. Now execute the statement using IS NOT NULL to retrieve the names of employees who do have phone extensions:

SELECT last_name, first_name, phone_ext FROM Employee
WHERE phone_ext IS NOT NULL

There should be 39 rows in the result set.

Advance To:

Comparing Against a Range or List of Values