sql script
select name, max(grade_sum) from (select user_id, sum(grade_num) as grade_sum from grade_info group by user_id order by grade_sum desc limit 1) tmp, `user` u where tmp.user_id=u.id
select name, max(grade_sum) from (select user_id, sum(grade_num) as grade_sum from grade_info group by user_id order by grade_sum desc limit 1) tmp, `user` u where tmp.user_id=u.id