解题思路：
1）先找出每个学校GPA最低分
2）然后再找出这个学校GPA最低分对应的id


select u2.device_id,u1.university,u1.gpa
from 
(select university,min(gpa) as gpa
from user_profile
group by university) as u1
left join 
user_profile as u2
on u1.university=u2.university and u1.gpa=u2.gpa
order by university;