MYSQL错误：You can't specify target table for update in FROM clause

这句话意思是：不能先select再更新（修改）同一个表。

可以再外嵌套多一层，这个问题只有mysql有，mssql和oracle都没有。

# 出错
delete from Person
where Id not in (
　　select min(Id) as Id from Person group by Email);

# 正确
delete from Person
where Id not in (
　　select Id from (
　　　　select min(Id) as Id from Person group by Email) p);