NULL Value

Use NULL to specify a value that is unknown, missing, or not applicable.

The NULL value is a special value that is different from any valid value for any data type. However, the NULL value is a legal value in any data type. These are two separate and distinct cases where NULL is used:

Situation	Description
missing	The field does have a value, but that value is unknown.
inapplicable	The field does not apply for this particular row.

SQL allows columns to be created with the NOT NULL restriction. This means that those particular columns cannot contain the NULL value.

The NULL value introduces the concept of three valued logic to SQL. The NULL value compared using any comparison operator with any value including the NULL value is UNKNOWN. The only search condition that returns TRUE is the IS NULL predicate. In SQL, rows are selected only if the search condition in the WHERE clause evaluates to TRUE; rows that evaluate to UNKNOWN or FALSE are not selected.

You can also use the IS [ NOT ] truth-value clause, where truth-value is one of TRUE, FALSE or UNKNOWN, to select rows where the NULL value is involved.

In the following examples, the column Salary contains the NULL value.

Condition	Truth value	Selected?
Salary = NULL	UNKNOWN	NO
Salary <> NULL	UNKNOWN	NO
NOT (Salary = NULL)	UNKNOWN	NO
NOT (Salary <> NULL)	UNKNOWN	NO
Salary = 1000	UNKNOWN	NO
Salary IS NULL	TRUE	YES
Salary IS NOT NULL	FALSE	NO
Salary = 1000 IS UNKNOWN	TRUE	YES

The same rules apply when comparing columns from two different tables. Therefore, joining two tables together does not select rows where any of the columns compared contain the NULL value.

The NULL value also has an interesting property when used in numeric expressions. The result of any numeric expression involving the NULL value is the NULL value. This means that if the NULL value is added to a number, the result is the NULL value—not a number. If you want the NULL value to be treated as 0, you must use the ISNULL( expression, 0 ) function.

Many common errors in formulating SQL queries are caused by the behavior of NULL. Be careful to avoid these problem areas. Note the effect of three-valued logic when combining search conditions.

Syntax

NULL

Usage

Anywhere

Permissions

Must be connected to the database

Side Effects

None

Example

The following INSERT statement inserts a NULL into the date_returned column of the Borrowed_book table.

INSERT
INTO Borrowed_book
( date_borrowed, date_returned, book )
VALUES ( CURRENT DATE, NULL, '1234' )