delete from user where id in (select id from user where id < 100)
这样写会报错：You can't specify target table 'student' for update in FROM clause
如果删除/更新和子查询使用同一张表会报上述错误
解决方法：起别名
delete from user where id in (select a.id from (select id from user where id < 100) a )
