学习目标：
sql
学习内容：
37.查询各科成绩最高分、最低分和平均分：
以如下形式显示：课程 ID,课程 name,最高分,最低分,平均分,及格率,中等率,优良率,优秀率
及格为>=60,中等为：70-80,优良为：80-90,优秀为：>=90
要求输出课程号和选修人数,查询结果按人数降序排列,若人数相同,按课程号升序排列
Select s.counm,c.name,count(stunm),max(tscore)ma,avg(tscore)a,min(tscore)m,sum(case when s.tscore>90 then 1 else 0 end)/count() as 优秀,sum(case when s.tscore>=80 and s.tscore<=90 then 1 else 0 end)/count() as 优良,sum(case when s.tscore>=70 and s.tscore<=80 then 1 else 0 end)/count() as 中等,sum(case when s.tscore>=60 then 1 else 0 end)/count() as 及格 from score s left join course c on s.counm=c.counm group by s.counm,c.name order by count(*) desc,s.counm asc;
38.按各科成绩进行排序,并显示排名, score 重复时保留名次空缺
select s.stunm,s.name,sc.tscore,c.name from students s left join score sc on s.stunm=sc.stunm left join course c on sc.counm=c.counm group by s.stunm,c.name,s.name,sc.tscore;
39.统计各科成绩各分数段人数：课程编号,课程名称,[100-85],[85-70],[70-60],[60-0] 及所占百分比
Select c.counm,c.name,
sum(case when s.tscore>=85 and s.tscore<=100 then 1 else 0 end) as “1”,
sum(case when s.tscore>=70 and s.tscore<85 then 1 else 0 end) as “2”,
sum(case when s.tscore>=60 and s.tscore<70 then 1 else 0 end)as “3”,
sum(case when s.tscore<60 then 1 else 0 end) as “4”
from score s left join course c on s.counm=c.counm group by c.counm,c.name
学习时间：
1月-3月，每天一小时左右
学习产出：
一周一发