Beginning SQL

Chapter 3
114
FirstName LastName DateOfBirth
Katie Smith 1977-01-09
Steve Gee 1967-10-05
John Jones 1952-10-05
Jenny Jones 1953-08-25
John Jackson 1974-05-27
Jack Johnson 1945-06-09
Seymour Botts 1956-10-21
Susie Simons 1937-01-20
Jamie Hills 1992-07-17
Stuart Dales 1956-08-07
William Doors 1994-05-28
Doris Night 1997-05-28
When the database looks at the records, it says for Catherine’s record, “Is NULL (unknown) less than
January 1, 1970, or is it greater than January 1, 1970?”
Well, the answer is unknown! Records that contain NULL values are always excluded from a results set.
The same principle applies to any comparison and to inner joins as well. Additionally, most database
systems consider unknowns equal when using an ORDER BY clause, so all NULL values are grouped
together.
In order to check for NULL values, you must use the IS NULL operator. To ensure that a value is not
NULL, use the IS NOT NULL operator, as in the following code:
SELECT FirstName, LastName, DateOfBirth
FROM MemberDetails
WHERE DateOfBirth ‘1970-01-01’
OR DateOfBirth IS NULL;
The preceding SQL returns all records, shown in the following table:
FirstName LastName DateOfBirth
Katie Smith 1977-01-09
Susie Simons 1937-01-20
John Jackson 1974-05-27
Steve Gee 1967-10-05
John Jones 1952-10-05