The result of null comparisons using the =
and !=
operators is inconsistent.
In the ANSI SQL standard, the value of NULL
is defined as unknown. It is not equal to anything, not even another NULL
value. Also, a null value is never not equal to another value. By default, T-SQL adopts the same behavior, but it can be turned off using the SET ANSI_NULLS OFF
command or setting the database ANSI_NULLS
option. In addition, certain database operations cannot or should not be performed if ANSI_NULLS
is turned off. Therefore, it is safest to make all T-SQL code ANSI compliant.
Example 1: The code in the following example will only print the message if the ANSI_NULLS
option has been turned off:
SET @x = NULLIF @x = NULL PRINT "ANSI NULLS OFF"
ANSI_NULLS
option is turned on:SET @x = 1
IF @x != NULL PRINT "ANSI NULLS ON"
[1] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 480
[2] Null Values: SQL Server 2008