题解 | 查询成绩

select
    count(*)
from
    (
        select
            s.sid
        from
            Student s
            join SC sc on s.sid = sc.sid
            join Course c on c.cid = sc.cid
            #where cname='语文' or cname='数学' or cname='英语'
        group by
            sc.sid
        having
            avg(sc.score) > 60
    ) s;

个人认为不需要where的条件，注释后确实仍可运行。