To specify a value that is unknown or not applicable
NULL
Anywhere
Must be connected to the database
None
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. The NULL value is used to represent missing or inapplicable information. 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. See “Search conditions” for a description of this clause.
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. See Chapter 4, “SQL Functions.”
Many common errors in formulating SQL queries are caused by the behavior of NULL. Be careful to avoid these problem areas. See “Search conditions” for a description of the effect of three-valued logic when combining search conditions.
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' )
