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根据多字段去重



有重复数据主要有一下几种情况:

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1.存在两条完全相同的纪录
这是最简单的一种情况,用关键字distinct就可以去掉
example: select distinct * from table(表名) where (条件)

2.存在部分字段相同的纪录(有主键id即唯一键)
如果是这种情况的话用distinct是过滤不了的,这就要用到主键id的唯一性特点及group by分组
example: select * from table where id in (select max(id) from table group by [去除重复的字段名列表,....])

eg:

SELECT count(1)
FROM
TCHotelLog.dbo.ELongRequestErrorLog erel WITH (NOLOCK)
LEFT JOIN TCHotelResource.dbo.HotelInfo hi WITH (NOLOCK)
ON hi.Id = erel.HotelId
LEFT JOIN TCHotelResource.dbo.RoomType rt WITH (NOLOCK)
ON rt.Id = erel.RoomTypeId
LEFT JOIN TCHotelResource.dbo.HotelPolicy hp WITH (NOLOCK)
ON hp.Id = erel.PolicyId
WHERE
erel.ErrorTime >= ‘2014-6-13‘
AND erel.RefId IN (9516406, 1646048)
AND erel.Id IN (SELECT max(Id)
FROM
TCHotelLog.dbo.ELongRequestErrorLog WITH (NOLOCK)
GROUP BY
MemberId
, PolicyId)



3.没有唯一键ID
这种情况我觉得最复杂,目前我只会一种方法,有那位知道其他方法的可以留言,交流一下:
example:select identity(int1,1) as id,* into newtable(临时表) from table
select * from newtable where id in (select max(id) from newtable group by [去除重复的字段名列表,....])
drop table newtable