mysql 去重留一 | 易学教程

首先先分析一下我们现在的目的是查询到这俩张表的所有数据然后进行删除重复记录每条数据只保留一条

第一步：
查询以下俩张表的重复记录 (关键字段重复>1)
ks_examcity 、 ks_examdistrict

select * from ks_examcity group by examSubjectID,city,province having count(examSubjectID)>1;
select * from ks_examdistrict group by examSubjectID,district,city having count(examSubjectID)>1;

第二步：
   查询这两张表中 每条记录的第一条记录 (每条记录重复中的第一条 id最小)
select min(id)
  from ks_examcity
 group by examSubjectID, city, province
having count(examSubjectID)> 1

SELECT min(id)
  FROM `ks_examdistrict`
 GROUP BY `examSubjectID`, `district`, `city`
HAVING COUNT(`examSubjectID`)> 1

第三步：
    联查： 查询所有的重复数据以及重复记录中第一条以外的数据
select `examSubjectID`
  from ks_examcity
 group by examSubjectID, city, province
having count(examSubjectID)> 1 )
   and id not in(
select min(id)
  from ks_examcity
 group by examSubjectID, city, province
having count(examSubjectID)> 1

SELECT `examSubjectID`
  from `ks_examdistrict`
 group by `examSubjectID`, `district`, `city`
HAVING COUNT(`examSubjectID`)> 1)
   and id not in(
SELECT min(id)
  FROM `ks_examdistrict`
 GROUP BY `examSubjectID`, `district`, `city`
HAVING COUNT(`examSubjectID`)> 1

第四步：  查询以上查询到的数据的所有id  并以查询到的id作为条件 进行删除

delete from `ks_examcity`  where id IN(  select id from (
select  id
  from ks_examcity
 where `examSubjectID` in(
select `examSubjectID`
  from ks_examcity
 group by examSubjectID, city, province
having count(examSubjectID)> 1 )
   and id not in(
select min(id)
  from ks_examcity
 group by examSubjectID, city, province
having count(examSubjectID)> 1)) as  tmpresult)

======================================================

DELETE
  FROM `ks_examdistrict`
 where id IN(
SELECT id
  from(
select id
  from `ks_examdistrict`
 where `examSubjectID` in(
SELECT `examSubjectID`
  from `ks_examdistrict`
 group by `examSubjectID`, `district`, `city`
HAVING COUNT(`examSubjectID`)> 1)
   and id not in(
SELECT min(id)
  FROM `ks_examdistrict`
 GROUP BY `examSubjectID`, `district`, `city`
HAVING COUNT(`examSubjectID`)> 1)) as tmpresult)

参考文章：https://www.cnblogs.com/jdbeyond/p/8157224.html

来源：51CTO

作者：zxxiong

链接：https://blog.51cto.com/13922718/2433185

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mysql去重

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