Question

mysql如何去除两个字段数据相同的记录

Answer 1

MySQL查询重复字段,及删除重复记录的方法
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)