MySQL 40题练习题和答案

2、查询“生物”课程比“物理”课程成绩高的所有学生的学号；
 思路：
    获取所有有生物课程的人（学号，成绩） - 临时表
    获取所有有物理课程的人（学号，成绩） - 临时表
    根据【学号】连接两个临时表：
        学号  物理成绩   生物成绩
 
    然后再进行筛选

select A.student_id, a, b
 　　from
  　　(select score.student_id, number as a from score left join course on course.cid = score.corse_id where cname='生物') as A
 　　left join
  　　(select score.student_id, number as b from score left join course on course.cid = score.corse_id where cname='物理') as B
 　　on
  　　A.student_id = B.student_id where a > if(isnull(b),0,b);

3、查询平均成绩大于60分的同学的学号和平均成绩；
思路：
        根据学生分组，使用avg获取平均值，通过having对avg进行筛选

select student_id, avg(number) from score group by student_id having avg(number)>60;

4、查询所有同学的学号、姓名、选课数、总成绩；

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

5、查询所姓李的老师个数

select count(tid) from teacher where tname like '波%';

6、查询没学过“叶平”老师课的同学的学号、姓名；
思路：
        先查到“李平老师”老师教的所有课ID
        获取选过课的所有学生ID
        学生表中筛选

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

7、查询学过“001”并且也学过编号“002”课程的同学的学号、姓名
思路：
        先查到既选择001又选择002课程的所有同学
        根据学生进行分组，如果学生数量等于2表示，两门均已选择
 法一：

select student.sid, student.sname from
 　　(select student_id, corse_id from score where corse_id=1 or corse_id=2) as B
 　　left join student on B.student_id = student.sid group by student_id having count(student_id)>1;

法二：

select student.sid, student.sname from score left join student on student.sid= student_id
　　 where corse_id= 1 or corse_id=2 group by student_id having count(corse_id)>1;

8、查询学过“叶平”老师所教的所有课的同学的学号、姓名
思路：
  同上,只不过将1和2变成in（老师课程）

select student_id, student.sname from score left join student on student.sid=student_id where corse_id in
 　　(select course.cid from course left join  teacher on course.teacher_id=teacher.tid where tname='波多')
 　　group by student_id having count(corse_id) = (select count(course.cid) from course left join teacher on
 　　course.teacher_id=teacher.tid where tname='波多');

9、查询课程编号“002”的成绩比课程编号“001”课程低的所有同学的学号、姓名；
思路：
  先找出分别找出课程1 和2 的成绩
  在连表 （id ，A.成绩，B.成绩）     比较成绩大小  
  找出学生id  然后用 in

　select student.sid, student.sname from student where sid in (
 　　select A.student_id as c from (select student_id,number  from score where corse_id=1) as A
 　　left join (select student_id, number from score where corse_id=2) as B on A.student_id=B.student_id
　　 where A.number> if(isnull(B.number),0,B.number)
);

10、查询有课程成绩小于60分的同学的学号、姓名；
思路： 
  先在score表找到成绩小于60的学生学号
  再用 sid in

select student.sid, student.sname from student where sid in (
 　　select  student_id from score where number<60
);

11、查询没有学全所有课的同学的学号、姓名；
思路：
  在分数表中根据学生进行分组，获取每一个学生选课数量
  如果数量==总课程数量，表示已经选择了所有课程

select student.sid from student where sid not in (
 　　select student.sid from score
 　　left join student on student_id=student.sid group by student_id having
 　　count(corse_id)=(select count(1) from course));

12、查询至少有一门课与学号为“001”的同学所学相同的同学的学号和姓名；
思路：
 法一：
        获取 001 同学选择的所有课程
        获取课程在其中的所有人以及所有课程
        根据学生筛选，获取所有学生信息
        再与学生表连接，获取姓名

 　　student.sid, student.sname from score left join student on score.student_id=student.sid
 　　where student_id !=1 and corse_id in (select corse_id from score where student_id=1);
    

法二：

 　　select student.sid, student.sname from
 　　(select * from score where corse_id in (select corse_id from score where student_id=1)) as A
 left join student on student.sid = A.student_id where student_id !=1;

13、查询至少学过学号为“001”同学所有课的其他同学学号和姓名；
思路：
 法一：
  同12
  先找到学过001的所有人
  然后个数=001所有的学科个数

select student.sid, student.sname from
 　　(select * from score where corse_id in (select corse_id from score where student_id=1)) as A
 　　left join student on  student.sid= A.student_id where A.student_id !=1 group by student_id having
 　　count(corse_id)=(select count(corse_id) from score where student_id=1) ;

法二：

select student_id,sname, count(course_id)
 　　from score left join student on score.student_id=student.sid
 　　where stduent_id !=1 and score_id in (select corse_id from score where student_id=1) group by student_id having
 count(corse_id)=(select count(corse_id) from score where student_id=1);

14、查询和“002”号的同学学习的课程完全相同的其他同学学号和姓名；
思路：
  先找到002学过的课程号
  在用 in 找到学过所有相同的课程号的人
  在通过连表，分组，计数 把相同的人找出来

　　 select student.sid,student.sname from  (select * from score where corse_id in (select corse_id from score where student_id=1)) as A
 　　left join student on student.sid = A.student_id where A.student_id!=2 group by A.student_id having
 　　count(A.student_id)= (select count(corse_id) from score where student_id=2);

15、删除学习“叶平”老师课的SC表记录；
思路：  
  先连表查姓名为叶平的老师 然后在删除

delete from score where corse_id in (
 　　select cid from course left join teacher on course.teacher_id=teacher.tid where teacher.tname='饭岛');

16、向SC表中插入一些记录，这些记录要求符合以下条件：①没有上过编号“002”课程的同学学号；②插入“002”号课程的平均成绩；
思路：
        由于insert 支持
                inset into tb1(xx,xx) select x1,x2 from tb2;
        所有，获取所有没上过002课的所有人，获取002的平均成绩

 　　insert into score(student_id, course_id, number) select sid,2,(select avg(number) from score where score_id=2)
 　　from student where sid not in (select stduent_id from score where score_id=2);

17、按平均成绩从低到高 显示所有学生的“语文”、“数学”、“英语”三门的课程成绩，按如下形式显示： 学生ID,语文,数学,英语,有效课程数,有效平均分；
思路： 
  
  连表查学生id,有效课程数,平均分
  通过已查询到的学生id为约束条件，再分别连表查询课程号们

 　　select student.sid,
 　　(select avg(number) from score left join course on course.cid = score.corse_id where course.cname="语文" and student_id = student.sid) as 语文,
 　　(select avg(number) from score left join course on course.cid = score.corse_id where course.cname="数学" and student_id = student.sid) as 数学,
 　　(select avg(number) from score left join course on course.cid = score.corse_id where course.cname="英语" and student_id = student.sid) as 英语,
 　　count(student_id)as 有效课程数,
 　　avg(number)as 平均成绩
 　　from score
 　　left join student on student.sid = student_id
 　　group by student_id order by avg(number) desc;

18、查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；
思路：
 法一：
  通过联表把课程表和成绩表关联
  再通过corse_id分组   找出最大最小成绩

select course.cid,max(number),min(number) from score left join course on course.cid = corse_id
 　　group by corse_id; 

法二：

 　　select corse_id,max(number),min(number) from score group by corse_id;

19、按各科平均成绩从低到高和及格率的百分数从高到低顺序；  
思路：
  case when .. then

　select
 　　corse_id,
 　　avg(number) as avgnum,
　　 sum(case when score.number > 60 then 1 else 0 END)/count(1)*100 as percent,
　　 from score group by corse_id order by avgnum asc, percent desc;

20、课程平均分从高到低显示（现实任课老师）；
思路：
  联表 分组

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

21、查询各科成绩前三名的记录:(不考虑成绩并列情况)

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

22、查询每门课程被选修的学生数；

　select count(student_id) from score group by corse_id;

23、查询出只选修了一门课程的全部学生的学号和姓名；

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

24、查询男生、女生的人数；
法一：

　select gender,count(gender) from student group by gender;

法二：

　　select * from (select count(1) as man from student where gender="男") as A,(select count(1) as women from student where gender="女")as B;

25、查询姓“张”的学生名单；

　select * from student where sname like '%蛋%';

26、查询同名同姓学生名单，并统计同名人数；

　select sname, count(sname) as '数量' from student group by sname;

27、查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列；

　　select corse_id, avg(if(isnull(number), 0, number)) as avg from score group by corse_id order by avg asc, corse_id desc;

28、查询平均成绩大于85的所有学生的学号、姓名和平均成绩；

select student.sid, student.sname, avg(number) from score left join student on student.sid = score.student_id
　　group by student_id having avg(number)>80;

29、查询课程名称为“生物”，且分数低于60的学生姓名和分数；

　　select * from course;
　　select student.sname, number from score
　　left join course on course.cid = corse_id
　　left join student on student.sid = student_id
　　where course.cname = "生物" and score.number < 60;

30、查询课程编号为003且课程成绩在80分以上的学生的学号和姓名；

　select student.sid, student.sname from score left join student on student.sid = student_id where corse_id = 3 and number >80;

31、求选了课程的学生人数
法一：

　select count(c) from(select count(student_id) as c from score group by student_id)as c;

法二：

　select count(distinct student_id) from score

32、查询选修“杨艳”老师所授课程的学生中，成绩最高的学生姓名及其成绩；
　

　select student.sname, number from score left join student on student.sid = score.student_id where score.corse_id in
　　select course.cid from course left join teacher on course.teacher_id = teacher.tid where tname="波多")
　　order by number desc limit 1;

33、查询各个课程及相应的选修人数；
　

　select course.cname,count(student_id) from score left join course on course.cid = corse_id
　　group by corse_id;

34、查询不同课程但成绩相同的学生的学号、课程号、学生成绩；

　　select DISTINCT s1.corse_id, s2.corse_id, s1.number from score as s1, score as s2 WHERE
　　s1.number = s2.number and s1.corse_id != s2.corse_id;

35、查询每门课程成绩最好的前两名；

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

36、检索至少选修两门课程的学生学号；

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

37、查询全部学生都选修的课程的课程号和课程名；

　select course.cid, course.cname from score left join course on course.cid = corse_id group by corse_id;

38、查询没学过“叶平”老师讲授的任一门课程的学生姓名；
　　

select student.sname from score left join student on student.sid = student_id where score.corse_id not in
　　(select tid from teacher where tname="波多") group by student_id;

39、查询两门以上不及格课程的同学的学号及其平均成绩；

　select student_id, avg(number) from score where number<60 group by student_id having count(student_id)>2;

40、检索“002”课程分数小于60，按分数降序排列的同学学号；

　select student_id from score where corse_id = 2 and number < 60 order by number desc;

41、删除“002”同学的“001”课程的成绩；
　　

delete from score where corse_id = 2 and student_id = 2;