在MYSQL里，不能先select一个表的记录，再按此条件进行更新和删除同一个表的记录

解决办法是，将select得到的结果，再通过中间表select一遍，这样就规避了错误，这个问题只出现于mysql，mssql和oracle不会出现此问题。

MySQL根据字段删除重复值

delete from sites where id not in (

select t.max_id from

(select max(id) as max_id from sites group by name,url) as t

);

Oracle根据字段删除重复值

delete from sites where id not in (select max(id) as max_id from sites group by name,url) as t);