各位大大求助 为什么不能加别名，一加别名就报未找到from关键字

Question

select a.s# ,a.sname,       sum(decode(c.cname,'语文',s.score,0)),       sum(decode(c.cname,'数学',s.score,0)),       sum(decode(c.cname,'英语',s.score,0)),       avg(s.score)from stu a,cou c,sc s where a.s# = s.s# and c.c# = s.c# group by a.s#,a.sname order by avg(s.score) desc