--创建一张允许NULL值的表
CREATE TABLE Test_NULL (
  num INT NOT NULL PRIMARY KEY
  ,fname NVARCHAR(50) NULL
  ,lname NVARCHAR(50) NULL
)

--对表插入4条数据：最后2条记录含有NULL值
INSERT INTO Test_NULL (num,fname,lname) VALUES(1, 'Tom','Jane')
INSERT INTO Test_NULL (num,fname,lname) VALUES(2, 'Dave','')
INSERT INTO Test_NULL (num,fname) VALUES(3, 'Aaron')
INSERT INTO Test_NULL (num,fname) VALUES(4, 'Betty')

SELECT
  *
FROM Test_NULL tn 
LEFT JOIN Test_NULL g
  ON tn.num = g.num
WHERE tn.lname = g.lname
------------------------------------------
1  Tom Jane  1  Tom Jane
2  Dave    2  Dave  

--查询lname为''的记录，即验证NULL不等于''
SELECT
  *
FROM Test_NULL tn
WHERE tn.lname = ''
------------------------------------------
2  Dave  

--通过IS NULL查询含有NULL的记录
SELECT
  *
FROM Test_NULL tn
WHERE tn.lname IS NULL
------------------------------------------
3  Aaron  NULL
4  Betty  NULL

--NULL不等于任何值，甚至NULL不等于NULL
--默认不能使用<>或=匹配NULL
SELECT
  *
FROM Test_NULL tn
WHERE tn.lname <> NULL OR tn.lname = NULL
------------------------------------------

--设置ANSI_NULLS为OFF，并使用=NULL查询记录
SET ANSI_NULLS OFF
SELECT
  *
FROM Test_NULL tn
WHERE tn.lname = NULL
------------------------------------------
3  Aaron  NULL
4  Betty  NULL

--插入1条含有NULL的新记录
INSERT INTO Test_NULL (num,fname,lname) VALUES(5, 'Serena', NULL)

--更新某条记录的字段值为NULL
UPDATE Test_NULL SET fname = NULL
WHERE num = 2