指定在与 Null 值一起使用等于 (=) 和不等于 (<>) 比较运算符时采用符合 ISO 标准的行为。
当 SET ANSI_NULLS 为 ON 时，即使 column_name 中包含空值，使用 WHERE column_name = NULL 的 SELECT 语句仍返回零行。即使 column_name 中包含非空值，使用 WHERE column_name <> NULL 的 SELECT 语句仍会返回零行。
当 SET ANSI_NULLS 为 OFF 时，等于 (=) 和不等于 (<>) 比较运算符不遵守 ISO 标准。使用 WHERE column_name = NULL 的 SELECT 语句返回 column_name 中包含空值的行。使用 WHERE column_name <> NULL 的 SELECT 语句返回列中包含非空值的行。
例子：
-- Create table t1 and insert values.
CREATE TABLE t1 (a INT NULL,b VARCHAR(40))
INSERT INTO t1 values (NULL,'HH')
INSERT INTO t1 values (0,'LL')
INSERT INTO t1 values (1,'OO')
GO
1、
-- Print message and perform SELECT statements.
PRINT 'Testing default setting';
DECLARE @varname int;
SELECT @varname = NULL;
SELECT * FROM t1 WHERE a = @varname;
SELECT * FROM t1 WHERE a <> @varname;
SELECT * FROM t1 WHERE a IS NULL;
GO
输出：
Testing default setting
a b
----------- ----------------------------------------
NULL HH
(1 row(s) affected)
a b
----------- ----------------------------------------
0 LL
1 OO
(2 row(s) affected)
a b
----------- ----------------------------------------
NULL HH
(1 row(s) affected)
2、
-- SET ANSI_NULLS to ON and test.
PRINT 'Testing ANSI_NULLS ON'
SET ANSI_NULLS ON
GO
DECLARE @varname int;
SELECT @varname = NULL;
SELECT * FROM t1 WHERE a = @varname;
SELECT * FROM t1 WHERE a <> @varname;
SELECT * FROM t1 WHERE a IS NULL;
GO
输出：
Testing ANSI_NULLS ON
a b
----------- ----------------------------------------
(0 row(s) affected)
a b
----------- ----------------------------------------
(0 row(s) affected)
a b
----------- ----------------------------------------
NULL HH
(1 row(s) affected)
3、
-- SET ANSI_NULLS to OFF and test.
PRINT 'Testing SET ANSI_NULLS OFF';
SET ANSI_NULLS OFF;
GO
DECLARE @varname int;
SELECT @varname = NULL;
SELECT * FROM t1 WHERE a = @varname;
SELECT * FROM t1 WHERE a <> @varname;
SELECT * FROM t1 WHERE a IS NULL;
GO
输出：
Testing SET ANSI_NULLS OFF
a b
----------- ----------------------------------------
NULL HH
(1 row(s) affected)
a b
----------- ----------------------------------------
0 LL
1 OO
(2 row(s) affected)
a b
----------- ----------------------------------------
NULL HH
(1 row(s) affected)