select
count(*)
from Student
where sId in (
    select
    sId
    from SC
    left join Course using(cId )
    where cname in(
        '语文','数学','英语'
    )
    group by 1
    having avg(score) > 60
)

where 子查询，但是问题是，不能连接别的表，如果外层主表是SC的话数据就重复了