1 单字段重复

生成零时表，其中uid是需要去重的字段

create table tmp_uid as (select uid from user_info group by uid having count(uid))

create table tmp_id as (select min(id) from user_info group by uid having count(uid))

数据量大时一定要为uid创建索引

create index index_uid on tmp_uid

create index index_id on tmp_id

删除多余的重复记录，保留重复项中id最小的

delete from user_info
where id not in (select id from tmp_id)
and uid in (select uid from tmp_uid)

2.多字段重复

由uid的重复间接的导致了relationship中的记录重复，故继续去重。先介绍正常处理流程，在介绍本人根据自身数据特点实践的更加有效的方法！

2.1一般方法

基本的同上面：

生成零时表

create table tmp_relation as (select source,target from relationship group by source,target having count(*)>1)

create table tmp_relationship_id as (select min(id) as id from relationship group by source,target having count(*)>1)

创建索引

create index index_id on tmp_relationship_id

删除

delete from relationship
where id not in (select id from tmp_relationship_id)
and (source,target) in (select source,target from relationship)

2.2 实践出真知

实践中发现上面的删除字段重复的方法，由于没有办法为多字段重建索引，导致数据量大时效率极低，低到无法忍受。最后，受不了等了半天没反应的状况，本人决定，另辟蹊径。

考虑到，估计同一记录的重复次数比较低。一般为2，或3，重复次数比较集中。所以可以尝试直接删除重复项中最大的，直到删除到不重复，这时其id自然也是当时重复的里边最小的。

大致流程如下：

1）选择每个重复项中id最大的一个记录

create table tmp_relation_id2 as (select max(id) from relationship group by source,target having count(*)>1)

2）创建索引（仅需在第一次时执行）

create index index_id on tmp_relation_id2

3）删除重复项中id最大的记录

delete from relationship where id in (select id from tmp_relation_id2)

4）删除临时表

drop table tmp_relation_id2

重复上述步骤1），2），3），4），直到创建的临时表中不存在记录就结束（对于重复次数的数据，比较高效）

参考：

查询及删除重复记录的方法 http://wenku.baidu.com/view/d4b3f134b90d6c85ec3ac6ad.html

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)

来源：https://www.cnblogs.com/rainduck/archive/2013/05/15/3079868.html

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