WITH cte AS
(SELECT SUM(IF(type='add',grade_num,-grade_num))AS grade_sum
FROM grade_info 
GROUP BY user_id)

SELECT G.user_id,U.name,SUM(IF(type='add',G.grade_num,-G.grade_num)) AS 'grade_sum'
FROM grade_info G LEFT JOIN user U ON G.user_id = U.id 
GROUP BY G.user_id
HAVING grade_sum IN (SELECT MAX(grade_sum) FROM cte)