MySQL45题

数据表
--1.学生表 Student(SId,Sname,Sage,Ssex)
SId 学生编号,Sname 学生姓名,Sage 出生年月,Ssex 学生性别
--2.课程表 Course(CId,Cname,TId)
CId 课程编号,Cname 课程名称,TId 教师编号
--3.教师表 Teacher(TId,Tname)
TId 教师编号,Tname 教师姓名
--4.成绩表 SC(SId,CId,score)
SId 学生编号,CId 课程编号,score 分数
创建测试数据
学生表 Student：
导入数据方法：将以下 mysql 语句，完整复制到 workbench 语句窗口（或者是 mysql 的黑窗口），然后运行即可导入，不需要另外创建表，下面表的操作一样。这些语句第一条是创建表（create table），后面都是插入数据到表中（insert into table ）。
create table Student(SId varchar(10),Sname varchar(10),Sage datetime,Ssex varchar(10));
insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('09' , '张三' , '2017-12-20' , '女');
insert into Student values('10' , '李四' , '2017-12-25' , '女');
insert into Student values('11' , '李四' , '2017-12-30' , '女');
insert into Student values('12' , '赵六' , '2017-01-01' , '女');
insert into Student values('13' , '孙七' , '2018-01-01' , '女');
科目表 Course
create table Course(CId varchar(10),Cname nvarchar(10),TId varchar(10));
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');
教师表 Teacher
create table Teacher(TId varchar(10),Tname varchar(10));
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');
成绩表 SC
create table SC(SId varchar(10),CId varchar(10),score decimal(18,1));
insert into SC values('01' , '01' , 80);
insert into SC values('01' , '02' , 90);
insert into SC values('01' , '03' , 99);
insert into SC values('02' , '01' , 70);
insert into SC values('02' , '02' , 60);
insert into SC values('02' , '03' , 80);
insert into SC values('03' , '01' , 80);
insert into SC values('03' , '02' , 80);
insert into SC values('03' , '03' , 80);
insert into SC values('04' , '01' , 50);
insert into SC values('04' , '02' , 30);
insert into SC values('04' , '03' , 20);
insert into SC values('05' , '01' , 76);
insert into SC values('05' , '02' , 87);
insert into SC values('06' , '01' , 31);
insert into SC values('06' , '03' , 34);
insert into SC values('07' , '02' , 89);
insert into SC values('07' , '03' , 98);

1.查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数
select * from sc a left join student b
on a.sid=b.sid
inner join sc c
on a.cid !=c.cid and a.cid='01' and c.cid='02'
where a.score>c.score;

1.1.png

1.1. 查询同时存在" 01 "课程和" 02 "课程的情况

select * from sc a inner join sc b

on a.sid=b.sid

and a.cid = '01' and b.cid = '02';

1.2.png

1.2. 查询存在" 01 "课程但可能不存在" 02 "课程的情况(不存在时显示为 null )
select * from sc a left join sc b
on a.sid=b.sid
and b.Cid='02' where a.Cid='01';

1.2.png

1.3. 查询不存在" 01 "课程但存在" 02 "课程的情况
select * from sc
where sid not in (select sid from sc where Cid='01')
and Cid='02';

1.3.png

2.查询平均成绩大于等于 60 分的同学的学生编号和学生姓名和平均成绩
select s.Sid,Sname,avg(score) from student s left join sc
on s.Sid=sc.Sid
group by s.Sid having avg(score)>=60;

2.png

3.查询在 SC 表存在成绩的学生信息
select s.* from sc left join student s on sc.Sid=s.Sid;

3.png

4.查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩(没成绩的显示为null)
select s.sid,sname,count(cid),sum(score) from student s left join sc

on s.sid=sc.sid group by s.sid;

4646.png

4.1. 查有成绩的学生信息
select * from student a where
a.sid in (select sid from sc group by sid);

4.1.png

5.查询「李」姓老师的数量
select count(*) from teacher where tname like '李%';

5.png

6.查询学过「张三」老师授课的同学的信息

select s.* from sc left join student s on s.sid=sc.sid where cid in (select cid from
course where tid=(select tid from teacher where tname='张三')) group by sid;

6.png

7.查询没有学全所有课程的同学的信息
select a.,count(cid) from student a inner join sc b on a.sid=b.sid group by a.sid
having count(cid)<(select count(cid) from course)

7.png

8.查询至少有一门课与学号为" 01 "的同学所学相同的同学的信息
select a. from student a inner join (select * from sc where cid in (select cid from sc where sid="01") group by sid) b on a.sid=b.sid;

8.png

9.查询和" 01 "号的同学学习的课程完全相同的其他同学的信息
(没有1和2的。因为"IN"的作用可以筛选与条件完全相同，或者有其中一个的）
SELECT s.*
FROM student s
INNER JOIN sc
ON s.sid = sc.sid
WHERE s.sid NOT IN(
SELECT sid
FROM sc #筛选没有'01'同学的课程的同学sid
WHERE cid NOT IN (
SELECT cid
FROM sc
WHERE sid = '01'))
AND NOT s.sid = '01'
GROUP BY s.sid
HAVING COUNT(cid) = (
SELECT COUNT(1)
FROM sc
WHERE sid ='01'
GROUP BY sid);

9.png

10.查询没学过"张三"老师讲授的任一门课程的学生姓名
SELECT Sname FROM student WHERE sid NOT IN (
SELECT sid
FROM sc
WHERE cid IN(
SELECT CID
FROM course c
INNER JOIN teacher t
ON c.Tid = t.Tid
WHERE Tname='张三'));

10.png

11.查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
select s.sid,Sname,avg(score) from student as s
left join sc on s.sid=sc.sid where score<60
group by s.sid
having count(cid)>2;

11.png

12.检索" 01 "课程分数小于 60，按分数降序排列的学生信息
select * from student as s left join sc on s.sid=sc.sid
where cid='01' and score<60
order by score desc;

12.png

13.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 student a
LEFT JOIN (
SELECT sc.,ag
FROM sc
INNER JOIN (
SELECT sid,AVG(score) ag
FROM sc
GROUP BY sid) s2
ON sc.sid=s2.sid) b
ON a.sid = b.sid
ORDER BY ag DESC;

13.png

14.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 sc.cid,max(sc.score) as 最高分,min(sc.score) as 最低分,
avg(sc.score) as 平均分, count() as 选修人数,
sum(case when sc.score>=60 then 1 else 0 end)/count() as 及格率,
sum(case when sc.score>=70 and sc.score<80 then 1 else 0 end )/count() as 中等率,
sum(case when sc.score>=80 and sc.score<90 then 1 else 0 end)/count() as 优良率,
sum(case when sc.score>=90 and sc.score<100 then 1 else 0 end)/count() as 优秀率 from sc
group by sc.cid order by count(*) desc,sc.cid asc;

14.png

15.按各科成绩进行排序，并显示排名， Score 重复时也继续排名
select sid,cid,score, @rank:=@rank+1 as rn from sc ,(select @rank:=0) as t
order by score desc;

15.png

16.1. 查询学生的总成绩，并进行排名，总分重复时不保留名次空缺
select s.* ,
@rank:=if (@sco=scos,'',@rank+1) as rn,
@sco:=scos from
(select sid,sum(score) as scos
from sc group by sid order by scos desc) s,
(select @rank:=0,@sco:=null)as t;

扩展：赋值变量
第一种方法：
set @a：=1；
第二种：
select @b:=2;
修改：select @b:=4;

select s.* ,
@rank:=if (@sco=scos,@rank,@rank+1) as rn,
@sco:=scos from
(select sid,sum(score) as scos
from sc group by sid order by scos desc) s,
(select @rank:=0,@sco:=null)as t;

16.1.png

17.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 * ,
concat(sum(case when sc.score>=85 and sc.score<=100 then 1 else 0 end)/count()100,'%') as '[85-100]',
concat(sum(case when sc.score>=70 and sc.score<85 then 1 else 0 end)/count()100,'%') as '[70-85)',
concat(sum(case when sc.score>=60 and sc.score<70 then 1 else 0 end)/count()100,'%') as '[60-70)',
concat(sum(case when sc.score>=0 and sc.score<60 then 1 else 0 end)/count()100,'%') as '[0-60)'
from sc group by sc.cid;

17.png

18.查询各科成绩前三名的记录
select * from sc a where(select count(*) from sc b where a.cid=b.cid
and b.score > a.score)<3;

18.png

19.查询每门课程被选修的学生数
select cid,count(1) from sc
group by cid;

19.png

20.查询出只选修两门课程的学生学号和姓名
select b.sid,b.Sname from sc a ,student b
where a.sid=b.sid group by a.sid having count() = 2;

20.png

21.查询男生、女生人数
select count() ,ssex from student group by ssex;

21.png

查询名字中含有「风」字的学生信息
select * from student where Sname like '%风%';

22.png

23.查询同名同性学生名单，并统计同名同姓人数
select * from student left join (select Sname ,Ssex,count(1)同名人数 from student group by Sname,Ssex) as t1
on student.Sname=t1.Sname and student.Ssex=t1.Ssex
where t1.同名人数>1;

23.png
查询 1990 年出生的学生名单
select * from student where year(Sage)='1990';

24.png
查查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列
select cid,avg(score) as avg_score from sc group by cid
order by avg_score desc,cid asc;

25.png

查询平均成绩大于等于 85 的所有学生的学号、姓名和平均成绩
select s.sid,Sname,avg(score) ag from student s left join sc
on s.sid = sc.sid group by s.sid
having ag >= 85;

26.png
查询课程名称为「数学」，且分数低于 60 的学生姓名和分数
select sname,cname,score from sc inner join student s on s.sid=sc.sid and score < 60
inner join course c on sc.cid = c.cid and cname = '数学';

27.png

查询所有学生的课程及分数情况（存在学生没成绩，没选课的情况）
select * from student s left join sc on s.sid=sc.sid;

28.png
查询任何一门课程成绩在 70 分以上的姓名、课程名称和分数
select a.sname, c.cname, sc.score
from sc left join student a on sc.sid=a.sid
left join course c on sc.cid=c.cid
where sc.score>70;

29.png

查询存在不及格的课程
select distinct sc.cid from sc where score<60;

30.png
查询课程编号为 01 且课程成绩在 80 分以上的学生的学号和姓名
SELECT s.sid,sname
FROM student s
INNER JOIN sc
ON s.sid = sc.sid
AND cid = '01'
AND score >= 80;

31.png
求每门课程的学生人数
select cid,count(1) from sc group by cid;

32.png
假设成绩不重复，查询选修「张三」老师所授课程的学生中，成绩最高的学生信息及其成绩

select * from sc left join student s on s.sid=sc.sid
left join course c on sc.cid=c.cid
left join teacher d on c.tid=d.tid where Tname='张三'
order by sc.score desc limit 1;

33.png

假设成绩有重复的情况下，查询选修「张三」老师所授课程的学生中，成绩最高的学生信息及其成绩
select sid ,score,cid,tname,rn
from
(select *,@rank:= if (@score=score,@rank,@rank+1) as rn ,
@score:=score from
(select a.sid , a.score ,c.cid ,d.Tname from sc a
left join student b on a.sid=b.sid
left join course c on a.cid=c.cid
left join teacher d on c.tid=d.tid where d.Tname='张三'
order by a.score desc ) a ,
(select @score:=null,@rank:=0) b) c
where rn = 1;

34.png
查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩
select *
from sc a
inner join sc b
on a.sid=b.sid
where a.cid != b.cid
and a.score=b.score
group by a.sid,a.cid;

35.png
查询每门科目成绩最好的前两名
select sid ,cid,score,rank from (
select * ,
@rank:=if(@c_cid=cid, if (@sco=score,@rank,@rank+1),1) as rank,
@sco:=score,
@c_cid:=cid
from sc,
(select @sco:=null,@rank:=0,@c_cid:=null) b
order by cid,score desc) a
where rank<=2;

36.png

统计每门课程的学生选修人数（超过 5 人的课程才统计）
select ,count() from sc group by cid;

37.png
检索至少选修两门课程的学生学号
select ,count() from sc group by sid
having count(*)>=2;

38.png

39、查询选修了全部课程的学生信息

select * ,count()from student s
inner join sc on s.sid=sc.sid
group by s.sid
having count(cid)=(select count() from course);

39.png

查询各学生的年龄，只按年份来算
select *,
year(now())-year(sage) as age
from student;

40.png
按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一
法1：使用timestampdiff函数
select sid,sname,
timestampdiff(year,sage,now()) as age
from student;
解法二：
select *,
case when substr(sage,6,5)<substr(now(),6,5) then year(now())-year(sage)
when substr(sage,6,5)>=substr(now(),6,5) then year(now())-year(sage) -1
end as age
from student;

41.png

42.查询本周过生日的学生
week:查询今年的第几周，参数为从星期几开始算新的一周
select * ,
week(sage),
week(now())
from student
where week(sage)=week(now());

由于now（）一直变，可能出现没有数据集的生日日期，所以可以挑一个数据集出现的数据放入
例如：select * ,
week(sage),
week(now())
from student
where week(sage)=week("2020-05-20");

42.png

查询下周过生日的学生
select *,
week(sage),
week(now())
from student
where week(sage)=week(now())+1;
查询本月过生日的学生
select *,
month(sage),
month(now())
from student
where month(sage)=month(now());

43.png

45.查询下月过生日的学生
select *,
month(sage),
month(now())
from student
where month(sage)=month(now())+1;