查询学生平均成绩及其名次

[andayfour]

有张表sc(s_id,c_id,score);s_id学生，c_id课程，score 分数
要插平均成绩和名次

select 1+(select count(*) from
 (select s_id,avg(score) as 平均成绩 from sc group by s_id) as T1 where T1.平均成绩>T2.平均成绩
)as RANK,s_id,平均成绩
from (select s_id,avg(score) as 平均成绩 from sc group by s_id)
as T2 order by 平均成绩 desc;

这样可以查出来
可是

select count(T1.平均成绩)+1 as Rank from
(select s_id,avg(score) as 平均成绩 from sc group by s_id) as T1,
(select s_id,avg(score) as 平均成绩 from sc group by s_id) as T2
where T1.平均成绩>T2.平均成绩;

得到一个记录16，为什么呢，不都是让select s_id,avg(score) as 平均成绩 from sc group by s_id的平均成绩比较吗
为什么上面的那条可以呢