摘要：對于第一種重復(fù)，比較容易解決，使用就可以得到無重復(fù)記錄的結(jié)果集。如果該表需要刪除重復(fù)的記錄重復(fù)記錄保留條，可以按以下方法刪除發(fā)生這種重復(fù)的原因是表設(shè)計不周產(chǎn)生的，增加唯一索引列即可解決。

1、在面試的時候碰到一個 問題，就是讓寫一張表中有id和name 兩個字段，查詢出name重復(fù)的所有數(shù)據(jù)，現(xiàn)在列下：

select * from xi a where (a.username) in  (select username from xi group by username  having count(*) > 1)

2、查詢出所有數(shù)據(jù)進行分組之后，和重復(fù)數(shù)據(jù)的重復(fù)次數(shù)的查詢數(shù)據(jù)，先列下：

select  count(username) as "重復(fù)次數(shù)",username from xi group by username  having count(*)>1 order by username desc

3、一下為 查看別人的 結(jié)果，現(xiàn)列下：查詢及刪除重復(fù)記錄的方法大全

1、查找表中多余的重復(fù)記錄，重復(fù)記錄是根據(jù)單個字段（peopleId）來判斷

select * from people
where peopleId in (select  peopleId  from  people  group  by  peopleId  having  count(peopleId) > 1)

2、刪除表中多余的重復(fù)記錄，重復(fù)記錄是根據(jù)單個字段（peopleId）來判斷，只留有rowid最小的記錄

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)

另外，MySQL中使用上面的SQL會報錯：You can"t specify target table "XXX" for update in FROM clause,不能在修改表的同時去查詢同一張表，因此可以像下面的語句一樣，將查詢結(jié)果命名為一張臨時表來作為修改表數(shù)據(jù)的條件，以規(guī)避上述錯誤。

delete from user where user_id in(
    (select user_id from 
        (select c.* from cab_user c where c.extra_id in 
            (select  a.extra_id  from user a group  by  a.extra_id having  count(a.extra_id) > 1) and c.user_id not in 
            (select min(b.user_id) from  user b group by b.extra_id having count(b.extra_id )>1)
        ) d
    )
)

3、查找表中多余的重復(fù)記錄（多個字段）

select * from vitae a
where (a.peopleId,a.seq) in  (select peopleId,seq from vitae group by peopleId,seq  having count(*) > 1)

4、刪除表中多余的重復(fù)記錄（多個字段），只留有rowid最小的記錄

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、查找表中多余的重復(fù)記錄（多個字段），不包含rowid最小的記錄

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)

(二)
比方說
在A表中存在一個字段“name”，
而且不同記錄之間的“name”值有可能會相同，
現(xiàn)在就是需要查詢出在該表中的各記錄之間，“name”值存在重復(fù)的項；

Select Name,Count(*) From A Group By Name Having Count(*) > 1

如果還查性別也相同大則如下:

Select Name,sex,Count(*) From A Group By Name,sex Having Count(*) > 1

(三)
方法一declare @max integer,@id integerdeclare cur_rows cursor local for select 主字段,count(*) from 表名 group by 主字段 having count(*) >； 1open cur_rowsfetch cur_rows into @id,@maxwhile @@fetch_status=0beginselect @max = @max -1set rowcount @maxdelete from 表名 where 主字段 = @idfetch cur_rows into @id,@maxendclose cur_rowsset rowcount 0

方法二＂重復(fù)記錄＂有兩個意義上的重復(fù)記錄，一是完全重復(fù)的記錄，也即所有字段均重復(fù)的記錄，二是部分關(guān)鍵字段重復(fù)的記錄，比如Name字段重復(fù)，而其他字段不一定重復(fù)或都重復(fù)可以忽略。

　　1、對于第一種重復(fù)，比較容易解決，使用select distinct * from tableName　　就可以得到無重復(fù)記錄的結(jié)果集。　　如果該表需要刪除重復(fù)的記錄（重復(fù)記錄保留1條），可以按以下方法刪除select distinct * into #Tmp from tableNamedrop table tableNameselect * into tableName from #Tmpdrop table #Tmp　　發(fā)生這種重復(fù)的原因是表設(shè)計不周產(chǎn)生的，增加唯一索引列即可解決。

　　2、這類重復(fù)問題通常要求保留重復(fù)記錄中的第一條記錄，操作方法如下　　假設(shè)有重復(fù)的字段為Name,Address，要求得到這兩個字段唯一的結(jié)果集select identity(int,1,1) as autoID, * into #Tmp from tableNameselect min(autoID) as autoID into #Tmp2 from #Tmp group by Name,autoIDselect * from #Tmp where autoID in(select autoID from #tmp2)　　最后一個select即得到了Name，Address不重復(fù)的結(jié)果集（但多了一個autoID字段，實際寫時可以寫在select子句中省去此列）

(四)
查詢重復(fù)

select * from tablename where id in (select id from tablenamegroup by idhaving count(id) > 1)

文章轉(zhuǎn)自：http://blog.csdn.net/a1108501...