学生信息表里有学号，姓名，班级，性别。成绩表有学号，学科，成绩。班级有c1.c2.c3三个，学科有s1.s2.s3三个。
统计各班各学科的成绩表，输出班级，学科，成绩。
列出各班各学科单科成绩小于60分的，输出班级，学科，学号，姓名，成绩。
列出登记有学生信息，但是没有考试成绩的学生名单，输出班级，学号，姓名。
统计各班各学科的评论成绩，输出班级，学科，平均成绩，按照平均成绩由高到低排列
1\select a.bj,b.xk,b.score from stu a
left inner join sco b on b.xh=a.xh
group by a.bj,b.xk
2、select a.xh,a.bj,b.xk,b.score from stu a
left inner join sco b on b.xh=a.xh
where b.score<'60'
3\select dinstinct a.xh,a.bj,b.xm from stu a
left inner join sco b on b.xh=a.xh
where b.score is NULL
3\select a.bj,b.xk,avg(b.score) c1 from stu a
left inner join sco b on b.xh=a.xh
group by a.bj,b.xk
order by c1 desc
1、
select a.bj, b.xk, avg(b.cj) as pjcj
from xsxxb a
inner join cjb b
on a.xh = b.xh
group by a.bj, b.xk
order by a.bj, b.xk;
2、
select a.bj, b.xk, a.xh, a.xm, b.cj
from xsxxb a
inner join cjb b
on a.xh = b.xh
where b.cj < 60
order by a.bj, b.xk, a.xh;
3、
select a.bj, a.xh, a.xm
from xsxxb a
left join cjb b
on a.xh = b.xh
where b.xh is null;
4、
select a.bj, b.xk, avg(b.cj) as pjcj
from xsxxb a
inner join cjb b
on a.xh = b.xh
group by a.bj, b.xk
order by b.xk, avg(b.cj) desc;