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Mysql删除重复记录,保留id最小的一条

2017-02-08 22:53  youxin  阅读(...)  评论(...编辑  收藏

 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,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。
========================
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)

(上面这条语句在mysql中执行会报错:


执行报错:1093 - You can't specify target table 'student' for update in FROM clause
原因是:更新数据时使用了查询,而查询的数据又做了更新的条件,mysql不支持这种方式。oracel和msserver都支持这种方式。
怎么规避这个问题?
再加一层封装,

delete from php_user where
username in (select username from ( select username from php_user group by username having count(username)>1) a)
and id not in ( select min(id) from (select min(id) as id from php_user group by username having count(username)>1 ) b)

 注意select min(id) 后面要有as id.

 

 

 

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

)

 

ref: http://www.greensoftcode.net/techntxt/2013120131251921893458
原文:http://bbs.ahpal.com/thread-3585-1-1.html