mysql多表查詢練習

導入hellodb.sql生成數據庫，並進入到數據庫之中

mysql -uroot < hellodb.sql
use mysql

(1)以ClassID分組，顯示每班的同窗的人數
`select classid,count(stuid) from students group by classid``

(2)以Gender分組，顯示其年齡之和
select gender,sum(age) from students group by gender

(3)以ClassID分組，顯示其平均年齡大於25的班級
select classid,avg(age) as new_age from students group by classid having new_age>25

(4)以Gender分組，顯示各組中年齡大於25的學員的年齡之和
select gender,sum(age) from students group by gender

(5)顯示前5位同窗的姓名、課程及成績
select name,course,score from (select name,score,courseid from (select * from students where stuid<=5) as s inner join scores on scores.stuid=s.stuid)as t inner join courses on courses.courseid=t.courseid

(6)顯示其成績高於80的同窗的名稱及課程
select name,course from (select name,score,courseid from (select from scores where Score>80) as t inner join students on students.stuid=t.stuid) as t inner join courses on courses.courseid=t.courseid

(7)求前8位同窗每位同窗本身兩門課的平均成績，並按降序排列
select t.stuid,avg(score) from (select stuid,courseid from (select from students where stuid<=8) as s inner join coc on s.classid=coc.classid) as t inner join scores on s cores.stuid=t.stuid group by t.stuid

(8)取每位同窗各門課的平均成績，顯示成績前三名的同窗的姓名和平均成績
select name,avg(score) as 平均分數 from (select name,courseid from students inner join coc on students.classid=coc.classid) as s inner join scores on s.courseid=scores.courseid group by stuid order by 平均分數 desc limit 3

(9)顯示每門課程課程名稱及學習了這門課的同窗的個數
select courseid,count(CourseID) from students inner join coc on coc.classid=students.classid group by courseid

(10)顯示其年齡大於平均年齡的同窗的名字
select * from students where age>(select avg(age) from students)

(11)顯示其學習的課程爲第一、2，4或第7門課的同窗的名字
select name,courseid from (select * from coc where CourseIDin ('1','2','4','7')) as new inner join students on students.classid=new.classid

(12)顯示其成員數最少爲3個的班級的同窗中年齡大於同班同窗平均年齡的同窗
select * from (select name,classid,age from students) as s inner join (select new.classid,avg(age) as cc from (select classid from students group by ClassID having count(stuid) >= 3) as new inner join students on students.classid=new.classid group by new.classid) as a on a.classid=s.classid where cc<age;