Question

SQL语句查询空值问题，请高手解决

SELECT dbo.TB_READER.sName, dbo.TB_TA_DEVICE_EVENT_KEY.sEventName, dbo.TB_USER.sUserID, dbo.TB_EVENT_LOG.nDateTime,
DATEADD(ss, dbo.TB_EVENT_LOG.nDateTime, '1970-01-01') AS kqdatetime,
dbo.TB_EVENT_LOG.IsRead
FROM dbo.TB_EVENT_LOG
INNER JOIN
dbo.TB_READER ON dbo.TB_EVENT_LOG.nReaderIdn = dbo.TB_READER.nReaderIdn
INNER JOIN
dbo.TB_TA_DEVICE_EVENT_KEY ON dbo.TB_EVENT_LOG.nReaderIdn = dbo.TB_TA_DEVICE_EVENT_KEY.nReaderIdn
INNER JOIN
dbo.TB_USER ON dbo.TB_EVENT_LOG.nUserID = dbo.TB_USER.sUserID
WHERE (dbo.TB_TA_DEVICE_EVENT_KEY.nKeyIdn <> 3)
AND (dbo.TB_USER.sUserID <> 1)
AND (dbo.TB_USER.sUserID <> 2)
AND (dbo.TB_USER.sUserID <> 3)
--AND (dbo.TB_EVENT_LOG.IsRead = NULL)

#195[6RQ][10.174.117.40] IN 10761 1215432625 2008-07-07 12:10:25.000 NULL
#195[6RQ][10.174.117.40] OUT 10761 1215432625 2008-07-07 12:10:25.000 NULL
#195[6RQ][10.174.117.40] IN 10761 1215432641 2008-07-07 12:10:41.000 NULL
#195[6RQ][10.174.117.40] OUT 10761 1215432641 2008-07-07 12:10:41.000 NULL
#195[6RQ][10.174.117.40] IN 10761 1215441040 2008-07-07 14:30:40.000 NULL
#195[6RQ][10.174.117.40] OUT 10761 1215441040 2008-07-07 14:30:40.000 NULL

为什么 AND (dbo.TB_EVENT_LOG.IsRead = NULL)用上时，就什么也查不到
IsRead 下面值为NULL

Answer 1

IsRead 不是必须的填写的字段是吗?
而且你也没有写值,默认的它也就没值
你要知道类型的话,使用类型的默认值去添加AND (dbo.TB_EVENT_LOG.IsRead = NULL
等于后面的值,而不是Null,NULL不是值,也不是字符串,只是一个空的意思
比如说IsRead是字符串对吧,你在里面没给他值,默认他也没值,那你用IsRead Null的条件,SQL会当成字符串去处理NULL这个值而不是你所谓的空
自然查不出值了

Answer 2

同一楼。。。。。。SQL语句里NULL值不能用=，<>符号运算

应该用is null ,is not null

Answer 3

数据库中不是dbo.TB_EVENT_LOG.IsRead = NULL
而是dbo.TB_EVENT_LOG.IsRead is NULL