回顾MySQL查询语句经典实例4

#按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩

select * from score 
left join (
    select sid,avg(score) as avgScore from score 
    group by sid
    )r 
on score.sid = r.sid
order by avgScore desc;

#查询各科成绩最高分，最低分和平均分
#显示形式：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 score.cid,
max(score.score) as 最高分,
min(score.score) as 最低分,
AVG(score.score) as 平均分,
count(*) as 选修人数,
sum(case when score.score>=60 then 1 else 0 end )/count(*) as 及格率,
sum(case when score.score>=70 and score.score<80 then 1 else 0 end )/count(*) as 中等率,
sum(case when score.score>=80 and score.score<90 then 1 else 0 end )/count(*) as 优良率,
sum(case when score.score>=90 then 1 else 0 end )/count(*) as 优秀率 
from score
GROUP BY score.cid
ORDER BY count(*) DESC , score.cid ASC;

#查询学生总成绩，并进行排名，总分重复不保留名次空缺
#SQL中的变量

set @crank=0;

#查询语句

select q.sid, total, @crank := (@crank +1) as 名次 from(
select score.sid, sum(score.score) as total from score
group by score.sid
order by total desc)q;

#查询学生信息，所选课程信息，所选课程老师信息，所选课程成绩信息

select * from student s,course c,teacher t,score sc where
s.sid = sc.sid and sc.cid = c.cid and c.tid = t.tid;

回顾MySQL查询语句经典实例4

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