题目截图：
建表代码：
drop table if exists user_profile;
CREATE TABLE `user_profile` (
`id` int NOT NULL,
`device_id` int NOT NULL,
`gender` varchar(14) NOT NULL,
`age` int ,
`university` varchar(32) NOT NULL,
`gpa` float,
`active_days_within_30` int ,
`question_cnt` int ,
`answer_cnt` int
);
INSERT INTO user_profile VALUES(1,2138,'male',21,'北京大学',3.4,7,2,12);
INSERT INTO user_profile VALUES(2,3214,'male',null,'复旦大学',4.0,15,5,25);
INSERT INTO user_profile VALUES(3,6543,'female',20,'北京大学',3.2,12,3,30);
INSERT INTO user_profile VALUES(4,2315,'female',23,'浙江大学',3.6,5,1,2);
INSERT INTO user_profile VALUES(5,5432,'male',25,'山东大学',3.8,20,15,70);
INSERT INTO user_profile VALUES(6,2131,'male',28,'山东大学',3.3,15,7,13);
INSERT INTO user_profile VALUES(7,4321,'male',28,'复旦大学',3.6,9,6,52);
题目解法：
select up.device_id,upn.university,upn.gpa
from user_profile up right join
(select university,min(gpa) as gpa
from user_profile
group by university
) as upn
on up.university = upn.university and up.gpa = upn.gpa
order by upn.university
解法解析：
首先，用了group by的查询语句只能在select语句中放置group by后面的字段和函数语句，这样的话就无法在语句中添加别的不需要分组的字段，如果要添加的话，就只能将要分组的字段和排行的字段做成一个表，然后再采用右连接的方式进行处理了
