老師問個問題：

update user1 inner join user2 on user1.user_name=user2.user_name set user1.over='齊天大圣'?? 和update user1 a inner join (select a.user_name from user1 a inner join user2 b on a.user_name=b.user_name) set a.over='齊天大圣'?? 這個兩個語句都能實現(xiàn)同樣的功能，那它們的區(qū)別在哪里呢，或者說有什么效率的上差異沒？