select
    count(*)
from
    (select
        s.sid
    from
        SC s 
        join Course c on s.cid=c.cid
    where
        c.cname in ('语文','数学','英语')
    group by 
        s.sid 
    having
        avg(s.score)>60) t

select count(*)，原来可以不用命名