查询MySQL数据库中重复数据并删除

需求：删除表中重复的数据，保留一条记录，其余的删除

下面这条SQL语句的功能就是查询出单表中重复的数据并删除

DELETE FROM wz_outpatient WHERE id in(SELECT a.id FROM 
(SELECT id FROM wz_outpatient WHERE (SFXMDM, KSDM,RQ,HJJE ) 
IN ( SELECT SFXMDM, KSDM,RQ,HJJE FROM wz_outpatient GROUP BY SFXMDM, KSDM,RQ,HJJE HAVING count( * ) > 1 ) 
AND id NOT IN ( SELECT min( id ) FROM wz_outpatient 
GROUP BY SFXMDM, KSDM,RQ,HJJE HAVING count( * ) > 1 )) a); 

咱们现在开始一步步的拆解这段语句

• step1

SELECT SFXMDM, KSDM,RQ,HJJE FROM wz_outpatient GROUP BY SFXMDM, KSDM,RQ,HJJE HAVING count( * ) > 1

这最里层的SQL语句中，咱们根据一些条件查询了wz_outpatient表中的几个字段值，并加上了Having筛选了数据重复条目大于1的结果。

• step2

SELECT id FROM wz_outpatient WHERE (SFXMDM, KSDM,RQ,HJJE ) 
IN ( 上例) 
AND id NOT IN ( SELECT min( id ) FROM wz_outpatient 
GROUP BY SFXMDM, KSDM,RQ,HJJE HAVING count( * ) > 1 )

这个SQL语句里面有两个查询条件

1. 将上例的结果作为这次语句的in条件查询id
2. id 在第二个条件中将重复数据查询返回了之后,筛选出除了id最小之外的其他数据
   PS: 也可用MAX()功能查询除了id最大的之外的结果集
   这一步骤的结果就是查询了该表中的除了最小id之外的重复数据

• step3

SELECT a.id FROM  (上例) a

逻辑上其实可以忽略这一步骤直接拼接delete语句就可删除重复数据并保留一条id最小的数据.但是在MySQL中执行的话,会报如下错误

image.png

大致意思就是不可在直接查询表又对表进行更新
所以我们就加了step3,将查询结果做出中间过渡表a

• step4

DELETE FROM wz_outpatient WHERE id in(SELECT a.id FROM 
(SELECT id FROM wz_outpatient WHERE (SFXMDM, KSDM,RQ,HJJE ) 
IN ( SELECT SFXMDM, KSDM,RQ,HJJE FROM wz_outpatient GROUP BY SFXMDM, KSDM,RQ,HJJE HAVING count( * ) > 1 ) 
AND id NOT IN ( SELECT min( id ) FROM wz_outpatient 
GROUP BY SFXMDM, KSDM,RQ,HJJE HAVING count( * ) > 1 )) a); 

执行SQL,亲测有效
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