NULL

NULL 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. NULL is used to represent missing or inapplicable information. There are two separate and distinct cases where NULL is used:

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

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.

Column space utilization for NULL values is 1 bit per column and space is allocated in multiples of 8 bits. The NULL bit usage is fixed based on the number of columns in the table that allow NULL values.

The IS [ NOT ] truth-value clause, where truth-value is one of TRUE, FALSE or UNKNOWN can be used to select rows where the NULL value is involved. For a description of this clause, see Search conditions.

In the following examples, the column Salary contains NULL.

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.

NULL also has an interesting property when used in numeric expressions. The result of any numeric expression involving the NULL value is NULL. This means that if NULL is added to a number, the result is NULL—not a number. If you want NULL 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. You have to be careful to avoid these problem areas. For a description of the effect of three-valued logic when combining search conditions, see Search conditions.

In SQL, comparisons to NULL within search conditions yield UNKNOWN as the result. However, when determining whether or not two rows are duplicates of each other, SQL treats NULL as equivalent to NULL. These semantics apply to the set operators (UNION, INTERSECT, EXCEPT), GROUP BY, PARTITION within a WINDOW clause, and SELECT DISTINCT.

For example, if a column called redundant contained NULL for every row in a table T1, then the following statement would return a single row:

SELECT DISTINCT redundant FROM T1;

Must be connected to the database.

None.

• ansinull option
• tds_empty_string_is_null option

• Expressions
• 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' );