MySQL查询重复字段,及删除重复记录的方法

我的实例 代理统计表删除资讯重复记录

来源文章

mysql中You cant specify target table for update in FROM clause错误解决方法

我的实例 代理统计表删除资讯重复记录

-- select * from daili_workcount a
-- where (a.type,a.work_id) in (select type,work_id from daili_workcount group by type,work_id having count(*) > 1)
-- create table `tmptable` as (
-- select id,type,work_id from daili_workcount a
-- where (a.type,a.work_id) in (select type,work_id from daili_workcount group by type,work_id having count(*) > 1));
DELETE from daili_workcount WHERE id in(
select b.id from
(
select a.id  from daili_workcount a
where (a.type,a.work_id) in (select type,work_id from daili_workcount group by type,work_id having count(*) > 1)
and a.id not in (select min(id) from daili_workcount group by type,work_id having count(*)>1)
) b
)

来源文章：

[教学] MySQL查询重复字段,及删除重复记录的方法

MySQL, 数据库, 数据库, 字段, 服务器

数据库中有个大表，需要查找其中的名字有重复的记录id，以便比较。如果仅仅是查找数据库中name不重复的字段，很容易:

SELECT min(

id

),

name

FROM

table

GROUP BY

name

;

但是这样并不能得到说有重复字段的id值。（只得到了最小的一个id值）查询哪些字段是重复的也容易:

SELECT

name

,count(

name

) as count FROM

table

GROUP BY

name

HAVING count(

name

) >1 ORDER BY count DESC;

但是要一次查询到重复字段的id值，就必须使用子查询了，于是使用下面的语句。

SELECT

id

,

name

FROM

table

WHERE

name

in (

SELECT

name

FROM

table

GROUP BY

name

HAVING count(

name

) >1);

但是这条语句在mysql中效率太差，感觉mysql并没有为子查询生成零时表。于是使用先建立零时表:

create table

tmptable

as (

SELECT

name

FROM

table

GROUP BY

name

HAVING count(

name

) >1);

然后使用多表连接查询：

SELECT a.

id

, a.

name

FROM

table

a,

tmptable

t WHERE a.

name

= t.

name

;

结果这次结果很快就出来了。

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查询及删除重复记录的方法

(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

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

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）

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

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

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；

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

如果还查性别也相同大则如下:

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

(三)

方法一

declare @max integer,@id integer

declare cur_rows cursor local for select 主字段,count() from 表名 group by 主字段 having count() >； 1

open cur_rows

fetch cur_rows into @id,@max

while @@fetch_status=0

begin

select @max = @max -1

set rowcount @max

delete from 表名 where 主字段 = @id

fetch cur_rows into @id,@max

end

close cur_rows

set rowcount 0

　　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，二是部分关键词段重复的记录，比如Name字段重复，而其它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

select distinct * from tableName

　　就可以得到无重复记录的结果集。

　　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

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

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）

(四)

查询重复

select * from tablename where id in (

select id from tablename

group by id

having count(id) > 1

)

mysql中You can’t specify target table for update in FROM clause错误解决方法

mysql中You can’t specify target table for update in FROM clause错误的意思是说，不能先select出同一表中的某些值，再update这个表(在同一语句中)。 例如下面这个sql：

delete from tbl where id in

(

select max(id) from tbl a where EXISTS

(

select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1

)

group by tac

)

delete from tbl where id in
(
select a.id from
(
select max(id) id from tbl a where EXISTS
(
select 1 from tbl b where a.tac=b.tac group by tac HAVING count(1)>1
)
group by tac
) a
)

也就是说将select出的结果再通过中间表select一遍，这样就规避了错误。注意，这个问题只出现于mysql，mssql和oracle不会出现此问题。