MySQL練習題

題目來源於網絡，答案全爲博主手敲

一、表關係建立

表關係分析：

二、增刪查改

1.查詢“生物”課程比“物理”課程成績高的所有學生的學號

select * from 
(select student_id,course_id,score from score left join course on score.course_id = course.cid where course.cname='生物') as A
left join
(select student_id,course_id,score from score left join course on score.course_id = course.cid where course.cname='物理') as B
on A.student_id = B.student_id 
where A.score>B.score

2.查詢平均成績大於60分的同學的學號和平均成績

select student_id,avg(score)
from score 
GROUP BY student_id
HAVING avg(score)>60

3.查詢所有同學的學號、姓名、選課數、總成績

select student_id,student.sname,count(1),sum(score)
from score left join student on score.student_id=student.sid
group by student_id

4.查詢姓“波”的老師的個數

select count(1)
from teacher
where teacher.teacher like '波%'

5.查詢沒學過“波多”老師課的同學的學號、姓名

方法1：

select student.sid,student.sname from student WHERE sid not in(
select score.student_id from course
left join score
on score.course_id=course.cid
left join teacher
on course.teacher_id = teacher.tid
where teacher.teacher='波多'
)

方法2：

select student.sid,student.sname from student where student.sid not in(
select student_id from score
WHERE course_id in
(select cid from course 
left join teacher on course.teacher_id=teacher.tid
where teacher.teacher='波多')
)

6.查詢學過“1”並且也學過編號“2”課程的同學的學號、姓名

select A.student_id from
(select student_id,score from score where course_id=1) as A
left join
(select student_id,score from score where course_id=2) as B
on A.student_id = B.student_id
where A.score is not null and B.score is not null

7.查詢學過“波多”老師所教的所有課的同學的學號、姓名

select student_id,student.sname from score 
left join student
on score.student_id=student.sid
where score.course_id in
(select cid from course left join teacher on course.teacher_id=teacher.tid
where teacher.teacher = '波多')
GROUP BY student_id
HAVING count(1)=(select count(cid) from course left join teacher on course.teacher_id=teacher.tid
where teacher.teacher = '波多')

8.查詢課程編號“2”的成績比課程編號“1”課程低的所有同學的學號、姓名

select P.student_id,student.sname from 
(select M.student_id from 
(select student_id,course_id,score from score where course_id=1) as M
left join
(select student_id,course_id,score from score where course_id=2) as N
on M.student_id = N.student_id
where M.score>N.score ) as P
left join student on student.sid=P.student_id

9.查詢有課程成績小於60分的同學的學號、姓名

select DISTINCT student_id,student.sname from score
left join student on score.student_id=student.sid
where score.score<60

10.查詢沒有學全所有課的同學的學號、姓名

select student_id,student.sname from score 
left join student on score.student_id=student.sid
group BY student_id
having count(1)<(select count(1) from course)

11.查詢至少有一門課與學號爲“1”的同學所學相同的同學的學號和姓名

select DISTINCT student_id from score
where course_id in
(select course_id from score
where student_id = 1)

12.查詢至少學過學號爲“1”同學所選課程中任意一門課的其他同學學號和姓名

select DISTINCT student_id from score
where course_id in
(select course_id from score
where student_id = 1)
and student_id!=1

13.查詢和“2”號的同學學習的課程完全相同的其他同學學號和姓名

select student_id,student.sname from score 
left join student on score.student_id=student.sid
where student_id in
(select student_id from score
group by student_id 
having student_id !=2 and
count(course_id)=(select count(course_id) from score where student_id=2) 
)
and 
course_id in (select course_id from score where student_id=2)
group by student_id
having count(student_id) = (select count(course_id) from score where student_id=2)

分析：

由內向外：

1.      找到所有選課數與二號相同的學生的id

select student_id from score

group by student_id

having student_id !=2 and

count(course_id)=

(select count(course_id) from score where student_id=2)#二號學生選課數

)

2.      在score表篩選student_id，這個student_id要在第一步找到的id中，這就保證了篩選出來的學生選課數與2號相同。

3.      把course_id不在2號所選的course_id的記錄刪掉，那麼剩下的是所選course全在二號所選課中的記錄

4.      分組，統計student_id出現的次數，如果次數與2號選課的數量相同，則表示該學生與2號學生選課完全一樣

14.刪除學習“Faker”老師課的SC表記錄

delete from score where course_id in
(select cid from course left join 
teacher on course.teacher_id=teacher.tid
where teacher.teacher='Faker')

15.向SC表中插入一些記錄，這些記錄要求符合以下條件：①沒有上過編號“2”課程的同學學號；②插入“2”號課程的平均成績

INSERT into score (student_id,course_id,score)
select sid,2,(select avg(score) from score where course_id=2 group by student_id)
from student where sid not in
(select student_id from score where course_id = 2)

16.按平均成績從低到高顯示所有學生的“語文”、“數學”、“英語”三門的課程成績，按如下形式顯示： 學生ID,LOL,性教育,體育,有效課程數,有效平均分

select sc.student_id,
	(select score from score left join course on score.course_id=course.cid where course.cname="LOL" and 
score.student_id=sc.student_id) as lo,
	(select score from score left join course on score.course_id=course.cid where course.cname="性教育" 
and score.student_id=sc.student_id) as xing,
	(select score from score left join course on score.course_id=course.cid where course.cname="體育" 
and score.student_id=sc.student_id) as Ty,
	count(sc.course_id),
	AVG(sc.score)
from score as sc
group by student_id desc

17.查詢各科成績最高和最低的分：以如下形式顯示：課程ID，最高分，最低分

select course_id,max(score),min(score) from score
group by course_id

18.按各科平均成績從低到高和及格率的百分數從高到低順序(case when score.score>60 then 1 else 0 end )相當於if else 判斷，如果score.score>60就返回1，否則返回0

select course_id,avg(score), 
sum(case when score.score>60 then 1 else 0 END)/count(1)*100 as percent
from score
group BY course_id

19.課程平均分從高到低顯示（顯示任課老師）

select avg(if(isnull(score.score),0,score.score)),teacher.teacher from course 
    left join score on course.cid = score.course_id 
    left join teacher on course.teacher_id = teacher.tid
 
group by score.course_id

if( isnull(score.score) , 0 , score.score) 如果條件成立返回0，否則返回score.score，條件爲score.score是否爲空

20.查詢各科成績前三名的記錄:(不考慮成績並列情況)，暫時沒做出來，貼一個網上的答案

select score.sid,score.course_id,score.num,T.first_num,T.second_num from score left join
    (
    select
        sid,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 0,1) as first_num,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 3,1) as second_num
    from
        score as s1
    ) as T
    on score.sid =T.sid
    where score.num <= T.first_num and score.num >= T.second_num

21.查詢每門課程被選修的學生數

select course_id,count(1)
from score 
group by course_id

T23
select student_id 
from score
GROUP BY student_id
having count(1)=1

22.查詢出只選修了一門課程的全部學生的學號

select student_id 
from score
GROUP BY student_id
having count(1)=1

23.查詢男生、女生的人數

select * from 
(select count(1) as man from student where sex='男') as A,
(select count(1) as woman from student where sex='女') as B

24.查詢姓“鋼”的學生名單

select student.sid,student.sname from student
where sname like '鋼%'

25.查詢同名同姓學生名單，並統計同名人數

select student.sname,count(1)
from student,(select sid,sname from student)as SC
where student.sid!=SC.sid and student.sname=SC.sname

26.查詢每門課程的平均成績，結果按平均成績升序排列，平均成績相同時，按課程號降序排列

select course_id,avg(score)
from score
group by course_id
ORDER BY score ASC,course_id desc

27.查詢平均成績大於85的所有學生的學號、姓名和平均成績

select student_id,student.sname,avg(score)
from score left JOIN student on score.student_id=student.sid
group by student_id
having avg(score)>85

28.查詢課程名稱爲“物理”，且分數低於60的學生姓名和分數

select student.sname,score from score left join student on score.student_id=student.sid
where course_id = (select cid from course where course.cname='物理')
and score < 60

29.查詢課程編號爲2且課程成績在80分以上的學生的學號和姓名

select student_id,student.sname from score left join student on score.student_id=student.sid
where course_id = 2 and score>80

30.求選了課程的學生人數

select count(1) from 
(select student_id from score 
group by student_id) as M

31.查詢選修“波多”老師所授課程的學生中，成績最高的學生姓名及其成績

select student.sname,max(score) from score left join 
student on score.student_id=student.sid
where course_id in 
(select cid from course left join teacher on course.teacher_id=teacher.tid where teacher.teacher='波多')

32.查詢各個課程及相應的選修人數

select course_id,count(1),M.cname from score left join
(select cid,cname from course) as M
on score.course_id=M.cid
group by course_id

33.查詢不同課程但成績相同的學生的學號、課程號、學生成績

select score.student_id,M.student_id,score.course_id,score.score from score left join 
(select student_id,score,course_id from score) as M
on score.score=M.score and score.student_id!=M.student_id
and score.course_id!=M.course_id

34.查詢每門課程成績最好的前兩名，同20

select score.sid,score.course_id,score.num,T.first_num,T.second_num from score left join
    (
    select
        sid,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 0,1) as first_num,
        (select num from score as s2 where s2.course_id = s1.course_id order by num desc limit 1,1) as second_num
    from
        score as s1
    ) as T
    on score.sid =T.sid
    where score.num <= T.first_num and score.num >= T.second_num

35.檢索至少選修兩門課程的學生學號

select student_id from score
group by student_id
having count(1)>2

36.查詢全部學生都選修的課程的課程號

select course_id from score group by course_id
having count(1)=(select count(1) from student)

37.查詢沒學過“波多”老師講授的任一門課程的學生姓名

select sid from student where sid not in(
select DISTINCT student_id from score
where course_id in
(select cid from course left join teacher on course.teacher_id=teacher.tid where teacher.teacher='波多')
)

38.查詢兩門以上不及格課程的同學的學號及其平均成績

select student_id from score 
group by student_id 
having count(case when score<60 then 1 ELSE null END)>2

注：0會被count計數，null不會

39.檢索“4”課程分數小於60，按分數降序排列的同學學號

select student_id,score 
from score
where course_id = 4 and score<60 
order by score DESC

40.刪除“2”同學的“1”課程的成績

delete from score 
where student_id = 2
and course_id = 1