Mysql按字段分组取最大值记录

在实际工作中，我们经常碰到这样的工作情况，取出用户订单中给定用户的最大单笔购买金额，此时，可以用到mysql的按字段分组取最大值，操作如下：

表(user_order)结构如下，我的操作是取出uid对应的最大的buy_time

方法0：

select uid,max(buy_time) from user_order group by uid

方法一：

select uid,buy_time from (select uid,buy_time from user_order order by buy_time desc) as a group by a.uid
方法二： select a.* from user_order as a where buy_time = (select max(buy_time) fromuser_order where a.uid=uid)
方法三： select a.* from user_order as a where not exists (select * from user_order where uid=a.uid_id and buy_time>a.buy_time)
方法四： select a.* from user_order as a where exists (select count(*) from user_order where uid=a.uid and buy_time>a.buy_time count(*)=0)