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