MYSQL为什么会报错？

代码：

select user_id,grade_num,rank() over(order by grade_num desc) rank from

    (select a.user_id user_id,sum(a.grade_num-case when b.grade_num is null then 0 else b.grade_num end) grade_num from
(select user_id,sum(grade_num) grade_num from grade_info where type='add' group by user_id)a
left join
(select user_id,sum(grade_num) grade_num from grade_info where type='reduce' group by user_id)b
on a.user_id=b.user_id 
group by a.user_id
    )a

报错信息：

第一个子查询中的case前面多了一个杠