• 参考答案
select u.id,u.name,sum(g.grade_num) grade_sum
from user u 
join grade_info g
on u.id=g.user_id
group by u.id
having grade_sum = (select sum(grade_num) r 
                    from grade_info g1 
                    group by user_id 
                    order by r desc 
                    limit 1)