题目

SQL query to delete all duplicate email entries in a table named Person, keeping only unique emails based on its smallest Id.

Id is the primary key column for this table.

For example, after running your query, the above Person table should have the following rows:

题目的意思是：从Person表中找出Email重复的记录，只保留Id最小的记录，删除其他的重复记录。

因为是先查找，再删除，所以我们也分两步进行:

1. 找出Email重复的记录

select p1.Id from Person as p1 inner join Person as p2 where p1.Email = p2.Email and p1.ID > p2.ID;

1. 删除重复记录

delete from Person where Id in ($sql1);

思路虽然是好的，但是这样写却是错误的：

delete from Person where Id in (select p1.Id from Person as p1 inner join Person as p2 where p1.Email = p2.Email and p1.ID > p2.ID);

错误信息为:

You can‘t specify target table ‘Person‘ for update in FROM clause

意思是：在MYSQL中，禁止在from子句中指定被更新的目标表。

所以，需要使用MYSQL规定的delete语法：

1. 从数据表t1中把那些id值在数据表t2里有匹配的记录全删掉:

delete t1 from t1,t2 where t1.id = t2.id;
or
delete from t1 using t1,t2 where t1.id = t2.id;

1. 从数据表t1里在数据表t2里没有匹配的记录查找出来并删除掉

delete t1 from t1 left join t2 on t1.id=t2.id where t2.id is null;

AC SQL

delete p1 from Person as p1 inner join Person as p2 on p1.ID > p2.ID and p1.Email = p2.Email;