方法一：
1、查找表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断
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select * from people where peopleId in ( select peopleId from people group by peopleId having count(peopleId)>1 ) |
2、删除表中多余的重复记录，重复记录是根据单个字段（peopleId）来判断，只留有rowid最小的记录
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delete from people where peopleId in ( select peopleId from people group by peopleId having count(peopleId)>1 ) and rowid not in ( select min(rowid) from people group by peopleId having count(peopleId)>1 ) |
3、查找表中多余的重复记录（多个字段）
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select * from vitae a where (a.peopleId,a.seq) in ( select peopleId,seq from vitae group by peopleId,seq having count(*)>1 ) |
4、删除表中多余的重复记录（多个字段），只留有rowid最小的记录
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delete from vitae a where (a.peopleId,a.seq) in ( select peopleId,seq from vitae group by peopleId,seq having count(*)>1 ) and rowid not in ( select min(rowid) from vitae group by peopleId,seq having count(*)>1 ) |
5、查找表中多余的重复记录（多个字段），不包含rowid最小的记录
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select * from vitae a where (a.peopleId,a.seq) in ( select peopleId,seq from vitae group by peopleId,seq having count(*)>1 ) and rowid not in ( select min(rowid) from vitae group by peopleId,seq having count(*)>1 ) |
比方说在A表中存在一个字段“name”，而且不同记录之间的“name”值有可能会相同，现在就是需要查询出在该表中的各记录之间，“name”值存在重复的项；
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Select Name,Count(*) From A Group By Name Having Count(*) >1 |
如果还查性别也相同大则如下:
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Select Name,sex,Count(*) From A Group By Name,sex Having Count(*)>1 |
方法二：
有两个意义上的重复记录，一是完全重复的记录，也即所有字段均重复的记录，二是部分关键字段重复的记录，比如Name字段重复，而其他字段不一定重复或都重复可以忽略。
1、对于第一种重复，比较容易解决，使用下面语句就可以得到无重复记录的结果集。
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select distinct * from tableName |
如果该表需要删除重复的记录（重复记录保留1条），可以按以下方法删除
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select distinct * into #Tmp from tableName drop table tableName select * into tableName from #Tmp drop table #Tmp |
发生这种重复的原因是表设计不周产生的，增加唯一索引列即可解决。
2、这类重复问题通常要求保留重复记录中的第一条记录，操作方法如下:
假设有重复的字段为Name,Address，要求得到这两个字段唯一的结果集
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select identity(int,1,1) as autoID, * into #Tmp from tableName select min(autoID) as autoID into #Tmp2 from #Tmp group by Name,autoID select * from #Tmp where autoID in ( select autoID from #tmp2 ) |
最后一个select即得到了Name，Address不重复的结果集（但多了一个autoID字段，实际写时可以写在select子句中省去此列）
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select * from tablename where id in ( select id from tablename group by id having count(id)>1 ) |
原文链接：mysql之如何删除表中的重复记录只保留其中一条，转载请注明来源！