问个成绩排序问题

我有表:
userid, credit
1,  12
2, 12
1,23
2,13
3,12该表是存储学生成绩
我想快速统计出该人得总成绩排名
于是这样写:
;with TempCreditRange(user_nm,rid) AS
(
  select userid , row_number() over(order by sum(credit) desc) as rid
     from user_course_reg
    group by userid
)
select @CreditRange = rid from TempCreditRange
where userid=@user_nm
但是数度很慢！谁能够帮助下改进！
xiexie!!!