MySQL SQL Training

　　源于知乎：50道SQL练习题

一、表数据

　　1、学生表——Student

create table Student(SId varchar(10),Sname varchar(10),Sage datetime,Ssex varchar(10));
insert into Student values('' , '赵雷' , '1990-01-01' , '男');
insert into Student values('' , '钱电' , '1990-12-21' , '男');
insert into Student values('' , '孙风' , '1990-05-20' , '男');
insert into Student values('' , '李云' , '1990-08-06' , '男');
insert into Student values('' , '周梅' , '1991-12-01' , '女');
insert into Student values('' , '吴兰' , '1992-03-01' , '女');
insert into Student values('' , '郑竹' , '1989-07-01' , '女');
insert into Student values('' , '张三' , '2017-12-20' , '女');
insert into Student values('' , '李四' , '2017-12-25' , '女');
insert into Student values('' , '李四' , '2017-12-30' , '女');
insert into Student values('' , '赵六' , '2017-01-01' , '女');
insert into Student values('' , '孙七' , '2018-01-01' , '女');

　　2、科目表——Course

create table Course(CId varchar(10),Cname nvarchar(10),TId varchar(10))
insert into Course values('' , '语文' , '');
insert into Course values('' , '数学' , '');
insert into Course values('' , '英语' , '');

　　3、教师表——Teacher

create table Teacher(TId varchar(10),Tname varchar(10))
insert into Teacher values('' , '张三');
insert into Teacher values('' , '李四');
insert into Teacher values('' , '王五');

　　4、成绩表——SC

create table SC(SId varchar(10),CId varchar(10),score decimal(18,1));
insert into SC values('' , '' , 80);
insert into SC values('' , '' , 90);
insert into SC values('' , '' , 99);
insert into SC values('' , '' , 70);
insert into SC values('' , '' , 60);
insert into SC values('' , '' , 80);
insert into SC values('' , '' , 80);
insert into SC values('' , '' , 80);
insert into SC values('' , '' , 80);
insert into SC values('' , '' , 50);
insert into SC values('' , '' , 30);
insert into SC values('' , '' , 20);
insert into SC values('' , '' , 76);
insert into SC values('' , '' , 87);
insert into SC values('' , '' , 31);
insert into SC values('' , '' , 34);
insert into SC values('' , '' , 89);
insert into SC values('' , '' , 98);

二、SQL练习题

1. 查询同时存在" 01 "课程和" 02 "课程的情况

/*只列出学生ID和课程情况*/
select t1.SId,t1.score as score01,t2.score as score02
from (select SId,score from sc where CId='') as t1,
    (select SId, score from sc   where CId='') as t2
where t1.SId=t2.SId;

+------+---------+---------+
| SId  | score01 | score02 |
+------+---------+---------+
| 01   |    80.0 |    90.0 |
| 02   |    70.0 |    60.0 |
| 03   |    80.0 |    80.0 |
| 04   |    50.0 |    30.0 |
| 05   |    76.0 |    87.0 |
+------+---------+---------+
5 rows in set (0.00 sec)

　　改进一下：

select sc1.sid,sc1.cid cid01,sc1.score score01,sc2.cid cid02,sc2.score score02
from sc sc1 join sc sc2
on sc1.sid=sc2.sid and sc1.cid="01" and sc2.cid="02";

+------+-------+---------+-------+---------+
| sid  | cid01 | score01 | cid02 | score02 |
+------+-------+---------+-------+---------+
| 01   | 01    |    80.0 | 02    |    90.0 |
| 02   | 01    |    70.0 | 02    |    60.0 |
| 03   | 01    |    80.0 | 02    |    80.0 |
| 04   | 01    |    50.0 | 02    |    30.0 |
| 05   | 01    |    76.0 | 02    |    87.0 |
+------+-------+---------+-------+---------+
5 rows in set (0.00 sec)

　　如果在此基础上列出更多关于学生的信息：

/*同时列出相关学生的信息*/
select t1.SId,t1.score as score01,t2.score as score02,t3.*
from (select SId,score from sc where CId='') as t1,
    (select SId, score from sc where CId='') as t2,
    Student as t3
where t1.SId=t2.SId and t1.SId=t3.SId;

+------+---------+---------+------+-------+---------------------+------+
| SId  | score01 | score02 | SId  | Sname | Sage                | Ssex |
+------+---------+---------+------+-------+---------------------+------+
| 01   |    80.0 |    90.0 | 01   | 赵雷  | 1990-01-01 00:00:00 | 男   |
| 02   |    70.0 |    60.0 | 02   | 钱电  | 1990-12-21 00:00:00 | 男   |
| 03   |    80.0 |    80.0 | 03   | 孙风  | 1990-05-20 00:00:00 | 男   |
| 04   |    50.0 |    30.0 | 04   | 李云  | 1990-08-06 00:00:00 | 男   |
| 05   |    76.0 |    87.0 | 05   | 周梅  | 1991-12-01 00:00:00 | 女   |
+------+---------+---------+------+-------+---------------------+------+
5 rows in set (0.00 sec)

1.1 查询" 01 "课程比" 02 "课程成绩高的学生的信息及课程分数

　　在“同时存在01课程和02课程”的基础上，添加一个条件即可：

select t1.SId,t1.score as score01,t2.score as score02,t3.* 
from (select SId,score from sc where CId='') as t1,(select SId, score from sc where CId='') as t2,Student as t3 
where t1.SId=t2.SId and t1.score>t2.score and t1.SId=t3.SId;

+------+---------+---------+------+-------+---------------------+------+
| SId  | score01 | score02 | SId  | Sname | Sage                | Ssex |
+------+---------+---------+------+-------+---------------------+------+
| 02   |    70.0 |    60.0 | 02   | 钱电  | 1990-12-21 00:00:00 | 男    |
| 04   |    50.0 |    30.0 | 04   | 李云  | 1990-08-06 00:00:00 | 男    |
+------+---------+---------+------+-------+---------------------+------+
2 rows in set (0.00 sec)

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 )

/*只列出学生ID*/
select * from (select SId ,score from sc where sc.CId='')as t1
    left join  (select SId ,score from sc where sc.CId='') as t2 on t1.SId=t2.SId;

/*列出学生具体信息*/
select t1.SId,t1.score as score01,t2.score as score02,t3.* from (select SId,score
    from sc where CId='') as t1 left join (select SId, score from sc where CId='') as t2 on  t1.SId=t2.SId,Student as t3
        where t1.SId=t3.SId;

　　结果如下：

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='') and CId='';

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

/*第一种*/
select t1.SId,t2.Sname,t1.avg
    from (select SId,AVG(score) as avg from sc group by SId having avg>=60) as t1,student as t2
        where t1.SId=t2.SId;

/*第二种*/
select student.*,t1.avgscore
    from student
        inner JOIN( select sc.SId ,AVG(sc.score)as avgscore from sc GROUP BY sc.SId HAVING AVG(sc.score)>=60)as t1 on student.SId=t1.SId;

3. 查询在 SC 表存在成绩的学生信息

/*第一种*/
select * from Student where SId in (Select SId from sc group by SId);

/*第二种*/
select DISTINCT student.* from student ,sc where student.SId=sc.SId;

/*第三种*/
select * from Student where SId in (Select DISTINCT SId from sc);

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 )

/*第一种*/
select stu.SId,stu.Sname,t1.NumCourse,t1.SumCourse
    from Student as stu left join (select SId, count(CId) as NumCourse,sum(score) as SumCourse from sc group by SId) as t1
        on stu.SId=t1.SId;

4.1 查看有成绩的学生信息

/*第一种*/
 select * from student where SId in (select distinct SId from sc);

/*第二种*/
select * from student where EXISTS(select * from sc where student.SId=sc.SId);

5. 查询「李」姓老师的数量

/*第一种*/
select count(*) from teacher where Tname like "李%";

6. 查询学过「张三」老师授课的同学的信息

/*第一种，嵌套查询，略复杂*/
 select * from Student
    where SId in
        (select SId from sc where CId in
            (select course.CId from course,teacher where course.TId=teacher.TId and teacher.Tname="张三"));

/*第二种*/
select student.* from teacher  ,course  ,student,sc
    where teacher.Tname='张三' and   teacher.TId=course.TId
        and   course.CId=sc.CId and   sc.SId=student.SId;

7. 查询没有学全所有课程的同学的信息

/*第一种*/
/*先找出休全了所有课的学生的ID*/
select * from student
　　where SId not in (
    　　select SId from sc group by SId having count(CId)=(select count(CId) from course));

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

/*第一种*/
/*01号学生列不列都可以*/
select * from student
where SId in
    (select distinct SId from sc where CId in
        (select distinct CId from sc where SId="01") and SId!="01") ;

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

/*第一种*/
/*sql过长的原因在于，同一个子查询（即t1,t2是同一个子查询），我不得不写两遍，没有找到好的解决方法*/
select * from student
where SId!="01" and SId in (
    select t1.SId
    from (
        select SId,group_concat(CId order by CId desc) allcourse
        from sc
        group by SId
        order by SId asc
    ) as t1
    where md5(t1.allcourse)=(
        select md5(t2.allcourse2)
        from (
            select SId,group_concat(CId order by CId desc) allcourse2
            from sc
            group by SId
            order by SId asc
        ) as t2
        where SId="01"
    )
) 

 /*第二种*/
select * from student
where student.SId !=''and student.SId not in (
    select t1.SId
    from (
        select student.SId,t.CId
        from student ,(select sc.CId from sc where sc.SId='') as t )as t1
        left join sc
        on t1.SId=sc.SId and t1.CId=sc.CId
        where sc.CId is null
); 

+------+-------+---------------------+------+
| SId  | Sname | Sage                | Ssex |
+------+-------+---------------------+------+
| 02   | 钱电  | 1990-12-21 00:00:00 | 男   |
| 03   | 孙风  | 1990-05-20 00:00:00 | 男   |
| 04   | 李云  | 1990-08-06 00:00:00 | 男   |
+------+-------+---------------------+------+
3 rows in set (0.00 sec)

10. 查询没学过"张三"老师讲授的任一门课程的学生姓名

/*第一种*/
select distinct * from student as stu
    where stu.SId not in (
        select SId from sc where CId in (select CId from course where TId=(
            select TId from teacher where Tname="张三"))) ;

/*第二种*/
select *
from student
where student.SId not in (
    select student.SId
    from student left join sc on student.SId=sc.SId
    where EXISTS (
        select *
        from teacher ,course
        where teacher.Tname='张三' and teacher.TId=course.TId and course.CId=sc.CId
    )
)

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

/*第一种*/
select t.SId,student.Sname,t.avgScore
from student,(select SId,count(CId) as numCourse,avg(score) as avgScore from sc where score<60 group by SId) as t
where t.SId=student.SId and numCourse>=2;

/*第二种*/
select student.SId,student.Sname,avg(sc.score)
from student ,sc
where student.SId=sc.SId and sc.score<60 GROUP BY sc.SId HAVING count(*)>=2;

12. 检索" 01 "课程分数小于 60，按分数降序排列的学生信息

select stu.*,sc.score
from student stu,sc
where stu.SId=sc.SId and CId="01" and score<60 order by score desc;

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

/*第一种*/
/*这里在order前加不加group by，结果都一样*/
select sc.*,t.avg
from sc ,(select SId,avg(score) avg from sc group by SId) as t
where sc.SId=t.SId
order by t.avg desc;

/*第二种*/
select sc.SId,sc.CId,sc.score,t1.avgscore
from  sc left join (select sc.SId,avg(sc.score) as avgscore from sc GROUP BY sc.SId) as t1 on sc.SId =t1.SId
ORDER BY t1.avgscore DESC;

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

/*注意group by，order by的位置，很困惑，与select要对应*/
select sc.CId ,co.Cname,
    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 and sc.score<80 then 1 else 0 end )/count(*)as 优良率,
    sum(case when sc.score>=90 then 1 else 0 end )/count(*)as 优秀率
from sc
left join course as co on sc.CId=co.CId
GROUP BY sc.CId
ORDER BY count(*)DESC,sc.CId asc

15. 按各科成绩进行排序，并显示排名。（三种形式都来一遍）（此时我新添加了2个学生的各三门成绩）

　　15.1 相同的score，rank不重复，按sid排名。所有的rank是连续的。（1，2，3）

　　from中的t表是为了变量初始化而无需单独的SET命令。

　　第二个when的条件总是true。

select sc.SId,sc.CId,sc.score,
    case
        when @pre_parent_code=sc.CId then @curRank:=@curRank+1
        when @pre_parent_code:=sc.CId then  @curRank:=1
    end as rank
from (select @curRank:=0,@pre_parent_code:='') as t ,sc
ORDER by sc.CId,sc.score desc;

+------+------+-------+------+
| SId  | CId  | score | rank |
+------+------+-------+------+
| 01   | 01   |  80.0 |    1 |
| 03   | 01   |  80.0 |    2 |
| 05   | 01   |  76.0 |    3 |
| 02   | 01   |  70.0 |    4 |
| 09   | 01   |  70.0 |    5 |
| 04   | 01   |  50.0 |    6 |
| 06   | 01   |  31.0 |    7 |
| 10   | 01   |  31.0 |    8 |
| 01   | 02   |  90.0 |    1 |
| 10   | 02   |  90.0 |    2 |
| 07   | 02   |  89.0 |    3 |
| 05   | 02   |  87.0 |    4 |
| 03   | 02   |  80.0 |    5 |
| 02   | 02   |  60.0 |    6 |
| 04   | 02   |  30.0 |    7 |
| 09   | 02   |  30.0 |    8 |
| 01   | 03   |  99.0 |    1 |
| 09   | 03   |  99.0 |    2 |
| 07   | 03   |  98.0 |    3 |
| 02   | 03   |  80.0 |    4 |
| 03   | 03   |  80.0 |    5 |
| 06   | 03   |  34.0 |    6 |
| 04   | 03   |  20.0 |    7 |
| 10   | 03   |  20.0 |    8 |
+------+------+-------+------+
24 rows in set (0.00 sec)

　　15.2 相同的Score，rank相同，而且所有rank是有间断的（例如第一和第二分数相同，那么前三名的rank为1，1，3）

　　这种排序应该是实际中会采纳的排序。

　　这种形式的排序比较好理解，self-join，找出另一个表中同组中值比自己大的行数，那自己的排名就是count+1,间断也是自动产生了。

select sc.SId,sc.CId,sc.score,tp.rank
from sc
left join (select SId, CId, (select count(sc2.score)+1 from sc sc2 where sc1.CId=sc2.CId and sc2.score>sc1.score) rank from sc sc1) tp
on sc.SId=tp.SId and sc.CId=tp.CId
order by sc.CId, rank

+------+------+-------+------+
| SId  | CId  | score | rank |
+------+------+-------+------+
| 01   | 01   |  80.0 |    1 |
| 03   | 01   |  80.0 |    1 |
| 05   | 01   |  76.0 |    3 |
| 02   | 01   |  70.0 |    4 |
| 09   | 01   |  70.0 |    4 |
| 04   | 01   |  50.0 |    6 |
| 06   | 01   |  31.0 |    7 |
| 10   | 01   |  31.0 |    7 |
| 01   | 02   |  90.0 |    1 |
| 10   | 02   |  90.0 |    1 |
| 07   | 02   |  89.0 |    3 |
| 05   | 02   |  87.0 |    4 |
| 03   | 02   |  80.0 |    5 |
| 02   | 02   |  60.0 |    6 |
| 04   | 02   |  30.0 |    7 |
| 09   | 02   |  30.0 |    7 |
| 01   | 03   |  99.0 |    1 |
| 09   | 03   |  99.0 |    1 |
| 07   | 03   |  98.0 |    3 |
| 02   | 03   |  80.0 |    4 |
| 03   | 03   |  80.0 |    4 |
| 06   | 03   |  34.0 |    6 |
| 04   | 03   |  20.0 |    7 |
| 10   | 03   |  20.0 |    7 |
+------+------+-------+------+
24 rows in set (0.00 sec)

　　经过自己对其它的参考，竟然改出一个简单的版本：

select sc2.sid as sid,sc2.cid as cid,sc2.score,if(isnull(sc3.score),1,count(*)+1) as rank
from sc sc2 left join sc sc3
on sc3.CId=sc2.CId and sc2.score<sc3.score
group by sc2.cid,sc2.sid
order by cid,score desc;
/*相比其它，我新插入了两个学生的供6条成绩*/
+------+------+-------+------+
| sid  | cid  | score | rank |
+------+------+-------+------+
| 01   | 01   |  80.0 |    1 |
| 03   | 01   |  80.0 |    1 |
| 05   | 01   |  76.0 |    3 |
| 02   | 01   |  70.0 |    4 |
| 09   | 01   |  70.0 |    4 |
| 04   | 01   |  50.0 |    6 |
| 06   | 01   |  31.0 |    7 |
| 10   | 01   |  31.0 |    7 |
| 01   | 02   |  90.0 |    1 |
| 10   | 02   |  90.0 |    1 |
| 07   | 02   |  89.0 |    3 |
| 05   | 02   |  87.0 |    4 |
| 03   | 02   |  80.0 |    5 |
| 02   | 02   |  60.0 |    6 |
| 04   | 02   |  30.0 |    7 |
| 09   | 02   |  30.0 |    7 |
| 09   | 03   |  99.0 |    1 |
| 01   | 03   |  99.0 |    1 |
| 07   | 03   |  98.0 |    3 |
| 02   | 03   |  80.0 |    4 |
| 03   | 03   |  80.0 |    4 |
| 06   | 03   |  34.0 |    6 |
| 04   | 03   |  20.0 |    7 |
| 10   | 03   |  20.0 |    7 |
+------+------+-------+------+
24 rows in set (0.00 sec)

15.3 相同的Score，rank相同，但是所有rank连续（第一第二分数相同，前四名1，1，2，3）

　　首先，这种排序是不科学的。

　　方法就是在上一题的基础上，在进行self-join时，添加"distinct"，去重即可。比如第三名，因为统计时有两个值比自己大，但是不管有几个相同的值比自己大，自己的rank都是2，用count(distinct col)+1就可以做到。

/**/
select sc.SId,sc.CId,sc.score,tp.rank
from sc
left join (
    select SId, CId, (
        select count(distinct sc2.score)+1
        from sc sc2
        where sc1.CId=sc2.CId and sc2.score>sc1.score
        ) rank
    from sc sc1
    ) tp
on sc.SId=tp.SId and sc.CId=tp.CId
order by sc.CId, rank

/**/
select sc2.sid as sid,sc2.cid as cid,sc2.score,if(isnull(sc3.score),1,count(distinct sc3.score)+1) as rank
from sc sc2 left join sc sc3
on sc3.CId=sc2.CId and sc2.score<sc3.score
group by sc2.cid,sc2.sid
order by cid,score desc;

+------+------+-------+------+
| sid  | cid  | score | rank |
+------+------+-------+------+
| 03   | 01   |  80.0 |    1 |
| 01   | 01   |  80.0 |    1 |
| 05   | 01   |  76.0 |    2 |
| 09   | 01   |  70.0 |    3 |
| 02   | 01   |  70.0 |    3 |
| 04   | 01   |  50.0 |    4 |
| 06   | 01   |  31.0 |    5 |
| 10   | 01   |  31.0 |    5 |
| 01   | 02   |  90.0 |    1 |
| 10   | 02   |  90.0 |    1 |
| 07   | 02   |  89.0 |    2 |
| 05   | 02   |  87.0 |    3 |
| 03   | 02   |  80.0 |    4 |
| 02   | 02   |  60.0 |    5 |
| 04   | 02   |  30.0 |    6 |
| 09   | 02   |  30.0 |    6 |
| 01   | 03   |  99.0 |    1 |
| 09   | 03   |  99.0 |    1 |
| 07   | 03   |  98.0 |    2 |
| 02   | 03   |  80.0 |    3 |
| 03   | 03   |  80.0 |    3 |
| 06   | 03   |  34.0 |    4 |
| 10   | 03   |  20.0 |    5 |
| 04   | 03   |  20.0 |    5 |
+------+------+-------+------+
24 rows in set (0.00 sec)

16. 查询学生的总成绩，并进行排名，总分重复时保留名次空缺（1，1，3）

select sc1.sid,sc1.sum,if(isnull(sc2.sum),1,count(sc2.sum)+1) rank
from (select sid,sum(score) as sum from sc group by sid) as sc1
left join (select sid,sum(score) as sum from sc group by sid) as sc2
on sc1.sum<sc2.sum
group by sc1.sid
order by rank asc

+------+-------+------+
| sid  | sum   | rank |
+------+-------+------+
| 01   | 269.0 |    1 |
| 03   | 240.0 |    2 |
| 02   | 210.0 |    3 |
| 09   | 199.0 |    4 |
| 07   | 187.0 |    5 |
| 05   | 163.0 |    6 |
| 10   | 141.0 |    7 |
| 04   | 100.0 |    8 |
| 06   |  65.0 |    9 |
+------+-------+------+
9 rows in set (0.00 sec)

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（1，1，2）

　　在上题基础上添加distinct即可。

select sc1.sid,sc1.sum,if(isnull(sc2.sum),1,count(distinct sc2.sum)+1) rank
from (select sid,sum(score) as sum from sc group by sid) as sc1
left join (select sid,sum(score) as sum from sc group by sid) as sc2
on sc1.sum<sc2.sum
group by sc1.sid
order by rank asc

（因为上几道排序的题，我在成绩表中添加了6行新的数据，这里再改回去，原表只有18行）

rename table sc to st;
rename table scback to sc;

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 course.CId,course.Cname,t1.*
from course LEFT JOIN (
    select sc.CId,
        sum(case when sc.score>=85 and sc.score<=100 then 1 else 0 end ) as '[85-100] Number',
        CONCAT(sum(case when sc.score>=85 and sc.score<=100 then 1 else 0 end )/count(*)*100,'%') as '[85-100]',

        sum(case when sc.score>=70 and sc.score<85 then 1 else 0 end ) as '[70-85) Number',
        CONCAT(sum(case when sc.score>=70 and sc.score<85 then 1 else 0 end )/count(*)*100,'%') as '[70-85)',

        sum(case when sc.score>=60 and sc.score<70 then 1 else 0 end ) '[60-70) Number',
        CONCAT(sum(case when sc.score>=60 and sc.score<70 then 1 else 0 end )/count(*)*100,'%') as '[60-70)',

        sum(case when sc.score>=0 and sc.score<60 then 1 else 0 end ) as '[0-60) Number',
        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
) as t1
on course.CId=t1.CId

18. 查询各科成绩前三名的记录

　　这里的前三名，有一定的歧义：

　　如果某科成绩如“100，100，95，95，90，90，88，...”，前三名是应该是指成绩为90，95，100的人6个学生？还是只有“100，90”的4个？

　　如果某科成绩为““100，100，100，95，95，90，90，88，...”，前三名是成绩为100的3个学生？还是成绩为“100，95，90”的7个学生？

select CId, SId, score
from sc
where (select count(*) from sc sc1 where sc1.CId=sc.CId and sc1.score>sc.score) <3
order by CId, score desc

19. 查询每门课程被选修的学生数

select course.*,t.count
from course
left join (select cid,count(sid) as count from sc group by cid) as t
on course.cid=t.cid;

20. 查询出只选修两门课程的学生学号和姓名

select sid,sname
from student
where sid in(select sid from sc group by sid having count(cid)=2);

21. 查询男生、女生人数

 select ssex,count(sid) from student group by ssex;

22. 查询名字中含有「风」字的学生信息

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

23. 查询同名同性学生名单，并统计同名人数

select s.sid,s.sname,s.ssex,count(*)
from student as s
join student as t
on s.sname=t.sname and s.ssex=t.ssex and s.sid!=t.sid
group by sid with rollup;

+------+-------+------+----------+
| sid  | sname | ssex | count(*) |
+------+-------+------+----------+
| 10   | 李四  | 女   |        1 |
| 11   | 李四  | 女   |        1 |
| NULL | 李四  | 女   |        2 |
+------+-------+------+----------+

24. 查询 1990 年出生的学生名单

select * from student where year(sage)=1990;

25. 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
from sc
group by cid order by avg desc;

+------+----------+
| cid  | avg      |
+------+----------+
| 02   | 72.66667 |
| 03   | 68.50000 |
| 01   | 64.50000 |
+------+----------+

select c.*,t.avg as avg
from course as c
join (select cid, avg(score) as avg from sc group by cid) as t
on c.cid=t.cid
order by avg desc,cid;

+------+-------+------+----------+
| CId  | Cname | TId  | avg      |
+------+-------+------+----------+
| 02   | 数学  | 01   | 72.66667 |
| 03   | 英语  | 03   | 68.50000 |
| 01   | 语文  | 02   | 64.50000 |
+------+-------+------+----------+

26. 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 stu.*
from student as stu
where sid in (select sid from sc group by sid having avg(score)>=85);

+------+-------+---------------------+------+
| SId  | Sname | Sage                | Ssex |
+------+-------+---------------------+------+
| 01   | 赵雷  | 1990-01-01 00:00:00 | 男    |
| 07   | 郑竹  | 1989-07-01 00:00:00 | 女    |
+------+-------+---------------------+------+

select stu.*,t.score
from student as stu,
    (select sid,avg(score) as score from sc group by sid having avg(score)>=85) as t
where stu.sid=t.sid;

+------+-------+---------------------+------+----------+
| SId | Sname | Sage | Ssex | score |
+------+-------+---------------------+------+----------+
| 01 | 赵雷 | 1990-01-01 00:00:00 | 男 | 89.66667 |
| 07 | 郑竹 | 1989-07-01 00:00:00 | 女 | 93.50000 |
+------+-------+---------------------+------+----------+

27. 查询课程名称为「数学」，且分数低于 60 的学生姓名和分数

select sid,sname
from student
where sid in (
    select sid
    from sc
    where cid in (
        select cid from course where cname="数学"
　　) and score<60
);

+------+-------+
| sid  | sname |
+------+-------+
| 04   | 李云  |
+------+-------+
1 row in set (0.00 sec)

28. 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 s.sid,s.sname,t.cid,t.score
from student as s
left join sc as t
on s.sid=t.sid
order by sid,cid;

29. 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 t.sid,s.sname,c.cname,t.score
from sc as t,student as s,course as c
where t.score>70 and t.cid=c.cid and t.sid=s.sid;

+------+-------+-------+-------+
| sid  | sname | cname | score |
+------+-------+-------+-------+
| 01   | 赵雷  | 语文  |  80.0 |
| 01   | 赵雷  | 数学  |  90.0 |
| 01   | 赵雷  | 英语  |  99.0 |
| 02   | 钱电  | 英语  |  80.0 |
| 03   | 孙风  | 语文  |  80.0 |
| 03   | 孙风  | 数学  |  80.0 |
| 03   | 孙风  | 英语  |  80.0 |
| 05   | 周梅  | 语文  |  76.0 |
| 05   | 周梅  | 数学  |  87.0 |
| 07   | 郑竹  | 数学  |  89.0 |
| 07   | 郑竹  | 英语  |  98.0 |
+------+-------+-------+-------+

30. 查询不及格的课程（什么意思？平均分不及格？）

select cid,avg(score) from sc group by cid having avg(score)<60;

31. 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 sid,sname from student
where sid in(
    select sid from sc where cid=01 and score>80
);

32. 求每门课程的学生人数

select cid ,count(sid) from sc group by cid;

33. 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

　　这里有多种情况需要考虑。

　　第一是一个老师只开一门课，那这课最高的分数只会有一个：

/*由于一个老师只开一门课，这里的in可以用=替换*/
select * from sc
where cid
in (select cid from course
      where tid=(select tid from teacher where tname="张三")
    )
order by score desc limit 1;

+------+------+-------+
| SId  | CId  | score |
+------+------+-------+
| 01   | 02   |  90.0 |
+------+------+-------+
1 row in set (0.00 sec)

　　骑士这里利用排序和LIMIT来输出是不对的，如果最高成绩有多名学生，那就错了，所以还是要从对cid分组入手，利用max函数找出该门课的最高分，再在成绩表中进行筛选。

　　第二是一个老师可以开多门课，那每一门课都会存在一个最高成绩，如果只是输出多门课中的一个最高成绩，那语句与上面情况一样（只是in不能用=替换）。

　　而如果是每一门课的最高成绩都输出，那么在成绩表sc中找出这个老师的所有课的成绩记录，然后对这个派生表用cid分组，并找出他的每门课的max(score)，然后再从成绩表sc入手找出cid相等，score=max(score)的记录。最后连接student表，输出信息与成绩。

　

34. 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

　　上面已经讨论过了！

35. 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

　　这里应该是指同一个学生有不同的课程成绩相同。

select distinct sc.*
from sc
join sc as t
on sc.sid=t.sid and sc.cid!=t.cid and sc.score=t.score;

36. 查询每门功课成绩最好的前两名

　　（这个取“前两名”可以利用排序和LIMIT解决了）

37. 统计每门课程的学生选修人数（超过 5 人的课程才统计）。

select cid, count(sid) as number
from sc
group by cid
having count(sid)>5;

+------+--------+
| cid  | number |
+------+--------+
| 01   |      6 |
| 02   |      6 |
| 03   |      6 |
+------+--------+
3 rows in set (0.00 sec)

38. 检索至少选修两门课程的学生学号

select sid ,count(cid) as cNum
from sc
group by sid
having cNum>1;

39. 查询选修了全部课程的学生信息

/*先查询总共共有多少门课*/
select count(distinct cid) from course;

+---------------------+
| count(distinct cid) |
+---------------------+
|                   3 |
+---------------------+

/*从成绩表找出满足条件的sid*/
select sid
from sc group by sid
having count(cid)=(select count(distinct cid) from course);
+------+
| sid  |
+------+
| 01   |
| 02   |
| 03   |
| 04   |
+------+
4 rows in set (0.00 sec)

40. 查询各学生的年龄，只按年份来算

select sid,sname,year(curdate())-year(sage) from student;

41. 按照出生日期来算，当前月日 < 出生年月的月日则，年龄减一

select student.SId,student.Sname,TIMESTAMPDIFF(YEAR,student.Sage,CURDATE()) as age
from student;

　　关于时间函数 TIMESTAMPDIFF(unit,datetime_expr1,datetime_expr2)

　　这里设置减法计算（expr2-expr1）后的unit为year，结果正好是符号题目的要求的

SELECT TIMESTAMPDIFF(YEAR,'2000-05-05','2002-05-05') as result;
+--------+
| result |
+--------+
|      2 |
+--------+

SELECT TIMESTAMPDIFF(YEAR,'2000-05-05','2002-05-06')  as result;
+--------+
| result |
+--------+
|      2 |
+--------+

SELECT TIMESTAMPDIFF(YEAR,'2000-05-05','2002-05-04')  as result;
+--------+
| result |
+--------+
|      1 |
+--------+

42. 查询本周过生日的学生

select * from student where week(sage)=week(now());

Empty set (0.00 sec)

43. 查询下周过生日的学生

select * from student where week(sage)=week(now())+1;

Empty set (0.00 sec)

44. 查询本月过生日的学生

select * from student where month(sage)=month(now());
Empty set (0.00 sec)

45. 查询下月过生日的学生

select * from student where month(sage)=month(now())+1;