月小升今天遇到的问题是students这个表有md这个字段重复。看看如何处理吧。
| 代码如下 | 复制代码 |
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select * from students where md in (select md from students group by md having count(md) > 1) order by md |
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注明，这个被group的字段，请索引，否则很慢
| 代码如下 | 复制代码 |
| delete from students where md in (select md from students group by md having count(md) > 1) and id not in (select min(id) from students group by md having count(md )>1) |
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这个语句在mysql下会报错
#1093 – You can’t specify target table ‘students’ for update in FROM clause
原因是好像mysql不准许我们进行联合删除内有条件语句指向自己的表。
策略是使用临时表，存储那些要删除的ID
| 代码如下 | 复制代码 |
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create table tmp (id int); insert into tmp (id) select id from students delete from students where id in (select id from tmp); |
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得出会被删除的数据
| 代码如下 | 复制代码 |
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select * from students where md in (select md from students group by md having count(md) > 1) and id not in (select min(id) from students group by md having count(md )>1) |
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得出过滤后的数据，不删除的数据。如果不用删除，此sql语句可以用来显示唯一数据
| 代码如下 | 复制代码 |
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select * from students where id in (select min(id) from students group by md having count(md )>1) |
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时间： 2024-07-29 23:02:04