查找表中多余的重复记录，重复记录是根据单个字段（DocId）来判断
select * from TableName
where DocId in (select DocId from TableName group by DocId having count(DocId) > 1)
例二：
select * from TableName
where UserName in (select UserName from TableName group by TableName having count(TableName) > 1 )
可以查出表中UserName相同的记录
2、删除表中多余的重复记录，重复记录是根据单个字段（DocId）来判断，只留有DocId最小的记录
delete from TableName
where DocId in (select DocId from TableName group by DocId having count(DocId) > 1)
and DocId not in (select min(DocId) from TableName group by DocId having count(DocId)>1)
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