28.查询所有学生的课程及分数情况（存在学生没成绩没选课的情况）
-- 【分析】给学生信息加上课程情况
select a.sid,a.sname,c.cname,b.score
from student a
left join sc b
on a.sid = b.sid
left join course c
on b.cid = c.cid;29.查询任何一门课程成绩在70分以上的姓名、课程名称和分数
-- 【分析】条件查询结果
-- 1、先找出成绩大于70分的成绩记录，这里面的sid就是要找的学生
select sid
from sc
where score > 70;
-- 2、和学生表进行关联得到学生信息记录
select b.sname,c.cname,a.score
from sc a
left join student b
on a.sid = b.sid
left join course c
on a.cid = c.cid
where a.score > 70;30.查询不及格的课程
-- 【分析】条件查询
-- 1、筛选出小于60分的成绩记录
select cid
from sc
where score < 60;
-- 2、去重得到cid，如果需要课程信息还可以和course表做关联
select a.*
from course a
where a.cid in (select cid
from sc
where score < 60);31.查询课程编号为01且课程成绩在80分以上的学生的学号和姓名
select a.sid,a.sname,b.cid,b.score
from student a
inner join sc b
on a.sid = b.sid
where b.cid = '01' and b.score > 80;32.求每门课程的学生人数
select b.cname,b.cid,count(a.sid) as cons
from sc a
left join course b
on a.cid = b.cid
group by a.cid;33.假设成绩不重复，查询选修[张三]老师所授课程的学生中，成绩最高的学生信息及成绩
-- 【分析】条件筛选查询，只需要一个人
select *
from sc a
left join student b
on a.sid = b.sid
left join course c
on a.cid = c.cid
left join teacher d
on c.tid = d.tid
where d.tname = '张三'
order by a.score desc
limit 1;34.假设成绩有重复，查询选修[张三]老师所授课程的学生中，成绩最高的学生信息及成绩
select *
from
(select
a.*,
case when @score=score then @rank
when @score:=score then @rank:=@rank + 1 end as rk
from
(select a.sid,a.score,b.sname,c.cid,d.tname
from sc a
left join student b
on a.sid = b.sid
left join course c
on a.cid = c.cid
left join teacher d
on c.tid = d.tid
where d.tname = '张三') a ,(select @score:=null,@rank:=0) t) s
where rk = 1;35.查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩
-- 【分析】条件关联筛选
select a.sid,a.cid,a.score
from sc a
inner join sc b
on a.sid = b.sid
where a.cid != b.cid and a.score = b.score
group by a.cid,a.sid;36.查询每门科目成绩最好的前两名
-- 【分析】用户变量排序
select
sid,cid,score,rk
from
(select
sc.*,
@rank:=if(@c_cid=cid,if(@sco=score,@rank,@rank+1),1) as rk,
@sco:=score,
@c_cid:=cid
from sc,(select @sco=null,@rank:=0,@c_cid:=null) b
order by cid,score desc) a
where a.rk<3;
