SELECT*from (
SELECT name,sum(grade_num) as grade_sum
FROM user as u join grade_info as g ON u.id=g.user_id
GROUP BY user_id
order BY grade_sum DESC) t
limit 1;

先把表进行连接，然后用limit 1选择第一个。
一定要注意连接生成的表一定要有别名。