select u.id,u.name,t.grade_num
from user u 
join (select user_id,sum(grade_num) grade_num from grade_info group by user_id) t 
on u.id = t.user_id
join (select sum(grade_num) maxg from grade_info group by user_id order by maxg desc limit 1) n 
on t.grade_num  = n.maxg
order by u.id