MYSQL学习总结3--mysql经典50例

-- 创建数据库
create database mysql_exercise_50;
use mysql_exercise_50;

-- 学生表
CREATE TABLE `Student`(
`s_id` VARCHAR(20),
`s_name` VARCHAR(20) NOT NULL DEFAULT '',
`s_birth` VARCHAR(20) NOT NULL DEFAULT '',
`s_sex` VARCHAR(10) NOT NULL DEFAULT '',
PRIMARY KEY(`s_id`)
)default charset=utf8 ;

--课程表
CREATE TABLE `Course`(
`c_id` VARCHAR(20),
`c_name` VARCHAR(20) NOT NULL DEFAULT '',
`t_id` VARCHAR(20) NOT NULL,
PRIMARY KEY(`c_id`)
)default charset=utf8 ;

--教师表
CREATE TABLE `Teacher`(
`t_id` VARCHAR(20),
`t_name` VARCHAR(20) NOT NULL DEFAULT '',
PRIMARY KEY(`t_id`)
)default charset=utf8 ;

--成绩表
CREATE TABLE `Score`(
`s_id` VARCHAR(20),
`c_id` VARCHAR(20),
`s_score` INT(3),
PRIMARY KEY(`s_id`,`c_id`)
)default charset=utf8 ;

--插入学生表测试数据
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('08' , '王菊' , '1990-01-20' , '女');
Student表：
+------+--------+------------+-------+
| s_id | s_name | s_birth    | s_sex |
+------+--------+------------+-------+
| 01   | 赵雷   | 1990-01-01 | 男    |
| 02   | 钱电   | 1990-12-21 | 男    |
| 03   | 孙风   | 1990-05-20 | 男    |
| 04   | 李云   | 1990-08-06 | 男    |
| 05   | 周梅   | 1991-12-01 | 女    |
| 06   | 吴兰   | 1992-03-01 | 女    |
| 07   | 郑竹   | 1989-07-01 | 女    |
| 08   | 王菊   | 1990-01-20 | 女    |
+------+--------+------------+-------+

# 课程表测试数据
insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');
Course表：
+------+--------+------+
| c_id | c_name | t_id |
+------+--------+------+
| 01   | 语文   | 02   |
| 02   | 数学   | 01   |
| 03   | 英语   | 03   |
+------+--------+------+

# 教师表测试数据
insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');
Teacher表：
+------+--------+
| t_id | t_name |
+------+--------+
| 01   | 张三   |
| 02   | 李四   |
| 03   | 王五   |
+------+--------+

# 成绩表测试数据
insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);
Score表：
+------+------+---------+
| s_id | c_id | s_score |
+------+------+---------+
| 01   | 01   |      80 |
| 01   | 02   |      90 |
| 01   | 03   |      99 |
| 02   | 01   |      70 |
| 02   | 02   |      60 |
| 02   | 03   |      80 |
| 03   | 01   |      80 |
| 03   | 02   |      80 |
| 03   | 03   |      80 |
| 04   | 01   |      50 |
| 04   | 02   |      30 |
| 04   | 03   |      20 |
| 05   | 01   |      76 |
| 05   | 02   |      87 |
| 06   | 01   |      31 |
| 06   | 03   |      34 |
| 07   | 02   |      89 |
| 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 st.*,sc.s_score as '语文',sc2.s_score as '数学' from student as st 
left join Score as sc on st.s_id=sc.s_id and sc.c_id='01' 
left join Score as sc2 on st.s_id = sc2.s_id and sc2.c_id='02'
where sc.s_score>sc2.s_score;

-- 2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
select st.*,sc.s_score as '语文',sc2.s_score as '数学' from student as st 
left join Score as sc on st.s_id=sc.s_id and sc.c_id='01' 
left join Score as sc2 on st.s_id = sc2.s_id and sc2.c_id='02'
where sc.s_score<sc2.s_score;

--3、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 st.s_id,st.s_name,round(avg(sc.s_score),2) as cjScore from student st
left join Score as sc on st.s_id=sc.s_id
group by st.s_id having cjScore >= 60; 

/*在mysql中，round函数用于数据的四舍五入，它有两种形式：
1、round(x,d)  ，x指要处理的数，d是指保留几位小数
这里有个值得注意的地方是，d可以是负数，这时是指定小数点左边的d位整数位为0,同时小数位均为0；
2、round(x)  ,其实就是round(x,0),也就是默认d为0；*/

--4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩(包括有成绩的和无成绩的)
select st.s_id,st.s_name,(case when round(avg(sc.s_score),2) is null then 0 else round(avg(sc.s_score),2) end ) as cjScore
from student as st left join Score as sc on st.s_id = sc.s_id 
group by st.s_id having cjScore<60 or cjScore is null;

--5、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
select st.s_id,st.s_name,count(sc.c_id) as '选课总数',
(case when sum(sc.s_score) is null or sum(sc.s_score)='' then 0 else sum(sc.s_score) end) as '总成绩'
from student st left join Score sc on st.s_id = sc.s_id group by st.s_id;

--6、查询"李"姓老师的数量 
select t.t_name as '老师名字',count(t.t_id) as '教师人数' from Teacher t group by t.t_id having t.t_name like "李%";

select t_name as '老师名字',count(t_id) as '教师人数' from Teacher where t_name like "李%";

--7、查询学过"张三"老师授课的同学的信息 
 select st.* from student st left join Score sc on st.s_id = sc.s_id 
    left join  course c on sc.c_id = c.c_id 
    left join Teacher t on c.t_id = t.t_id
    where t.t_name = '张三';

--8、查询没学过"张三"老师授课的同学的信息 
-- 张三老师教的课
select c.* from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三"
-- 有张三老师课成绩的st.s_id
 select sc.s_id from score sc where sc.c_id in (select c.c_id from course c left join teacher t on t.t_id=c.t_id where  t.t_name="张三")
-- 不在上面查到的st.s_id的学生信息,即没学过张三老师授课的同学信息
   select st.* from student st where st.s_id not in(
      select sc.s_id from score sc where sc.c_id in (select c.c_id from course c left join Teacher t on t.t_id=c.t_id where t.t_name = "张三")
  ) ; 


--9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
select st.* from student st 
inner join score sc on sc.s_id = st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
where st.s_id in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id
inner join course c2 on c2.c_id=sc2.c_id and c2.c_id="02"
)

select st.* from student st join score s on st.s_id=s.s_id and s.c_id='01'
 join score s2 on st.s_id=s2.s_id and s2.c_id='02';

 --10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
select st.* from student st 
inner join score sc on sc.s_id = st.s_id
inner join course c on c.c_id=sc.c_id and c.c_id="01"
where st.s_id not in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id
inner join course c2 on c2.c_id=sc2.c_id and c2.c_id="02"
)
或
select st.* from student st 
inner join score sc on sc.s_id = st.s_id and sc.c_id="01"
where st.s_id not in (
select st2.s_id from student st2 
inner join score sc2 on sc2.s_id = st2.s_id and sc2.c_id="02"
);

select st.* from  student st join score sc on st.s_id = sc.s_id and sc.c_id='01'
  where st.s_id not in(
     select st2.s_id from student st2 join score sc2 on st2.s_id=sc2.s_id and sc2.c_id='02'
  )

--11、查询没有学全所有课程的同学的信息
select * from student where s_id not in (
   select st.s_id from student st join score sc on st.s_id=sc.s_id and sc.c_id='01' 
   where st.s_id in (
      select st2.s_id from student st2 join score sc2 on st2.s_id=sc2.s_id and sc2.c_id='02'
   ) and st.s_id in(
      select st3.s_id from student st3 join score sc3 on st3.s_id=sc3.s_id and sc3.c_id='03'
   )
);
或
select * from student where s_id not in (
   select s_id from score group by s_id having count(c_id)=(select count(*) from course)
);

--12、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 distinct st.* from student st left join score sc on st.s_id = sc.s_id 
  where sc.c_id in (
     select sc2.c_id from student st2 left join score sc2 on st2.s_id=sc2.s_id where st2.s_id='01'
  );

select distinct st.* from student st join score s on st.s_id = s.s_id 
 where s.c_id = any(select c_id from score where s_id='01');

--13、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
select st.* from student st left join score sc on st.s_id=sc.s_id
group by st.s_id having  group_concat(sc.c_id)=(
   select group_concat(sc2.c_id) from student st2 left join score sc2
    on st2.s_id = sc2.s_id where st2.s_id='01'
);

-- 14、查询没学过"张三"老师讲授的任一门课程的学生姓名
select s_name from student  where s_id not in 
(
   select sc.s_id from score sc join course c on sc.c_id = c.c_id 
    join teacher t on c.t_id = t.t_id and t.t_name='张三'
);
或
select s_name from student st where s_id not in (
   select distinct st.s_id from student st left join score s on st.s_id=s.s_id where s.c_id in(
      select c_id from course where t_id =(
         select t_id from teacher where t_name='张三'
      )
   )
);



--15、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 st.s_id,st.s_name,avg(sc.s_score) from student st left join score sc on st.s_id=sc.s_id 
  where sc.s_id in (
     select sc.s_id from score sc where sc.s_score<60 or sc.s_score is null group by sc.s_id having count(sc.s_id)>=2
  ) group by st.s_id;
或
select st.s_id,st.s_name,avg(s.s_score) from student st left join score s on st.s_id=s.s_id where st.s_id in
(
   select s_id from score where s_score<60 or s_score is null group by s_id having count(s_score)>=2
)group by st.s_id,st.s_name ;


--16、检索"01"课程分数小于60，按分数降序排列的学生信息
select st.*,sc.s_score from student st 
inner join score sc on sc.s_id=st.s_id and sc.c_id="01" and sc.s_score<60
order by sc.s_score desc
或
select st.* ,sc.s_score from student st join score sc on st.s_id=sc.s_id
 where sc.c_id='01' and sc.s_score<60 order by sc.s_score desc;

 --17、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，可加round,case when then else end 使显示更完美
select st.s_id,st.s_name,sc.s_score "语文",sc2.s_score "数学" ,sc3.s_score "英语", avg(sc4.s_score) "平均分"
from student st left join score sc on st.s_id = sc.s_id and sc.c_id ="01"
left join score sc2 on st.s_id = sc2.s_id and sc2.c_id="02"
left join score sc3 on st.s_id = sc3.s_id and sc3.c_id="03"
left join score sc4 on st.s_id = sc4.s_id 
group by st.s_id 
order by avg(sc4.s_score) desc;
或
select st.s_id, st.s_name, avg(s.s_score) '平均成绩', s1.s_score '语文', s2.s_score '数学', s3.s_score '英语'
from student st 
left outer join score s on st.s_id = s.s_id 
left outer join score s1 on st.s_id = s1.s_id and s1.c_id = '01'
left outer join score s2 on st.s_id = s2.s_id and s2.c_id = '02'
left outer join score s3 on st.s_id = s3.s_id and s3.c_id = '03'
group by st.s_id, st.s_name
order by avg(s.s_score) desc;

--*18.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
select c.c_id,c.c_name,max(sc.s_score) "最高分",MIN(sc2.s_score) "最低分",avg(sc3.s_score) "平均分" 
,((select count(s_id) from score where s_score>=60 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "及格率"
,((select count(s_id) from score where s_score>=70 and s_score<80 and
 c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "中等率"
,((select count(s_id) from score where s_score>=80 and s_score<90 and
 c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优良率"
,((select count(s_id) from score where s_score>=90 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优秀率"
from course c
left join score sc on sc.c_id=c.c_id 
left join score sc2 on sc2.c_id=c.c_id 
left join score sc3 on sc3.c_id=c.c_id 
group by c.c_id

select c.c_id,c.c_name,max(sc.s_score) "最高分",MIN(sc.s_score) "最低分",avg(sc.s_score) "平均分" 
,((select count(s_id) from score where s_score>=60 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "及格率"
,((select count(s_id) from score where s_score>=70 and s_score<80 and
 c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "中等率"
,((select count(s_id) from score where s_score>=80 and s_score<90 and
 c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优良率"
,((select count(s_id) from score where s_score>=90 and c_id=c.c_id )/(select count(s_id) from score where c_id=c.c_id)) "优秀率"
from course c
left join score sc on sc.c_id=c.c_id 
group by c.c_id;

或
select c.c_id, c.c_name, max(s.s_score) '最高分', min(s.s_score) '最低分', avg(s.s_score) '平均分',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id ) 
    from score where c_id = c.c_id and s_score >= 60) '及格率',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id ) 
    from score where c_id = c.c_id and (s_score between 70 and 79)) '中等率',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id )  
    from score where c_id = c.c_id and (s_score between 80 and 89)) '优良率',
(select count(s_score)/(select count(s_score) from score where c_id = c.c_id ) 
    from score where c_id = c.c_id and s_score >= 90) '优秀率' 
from course c left outer join score s on c.c_id = s.c_id
group by c.c_id, c.c_name;


select c.c_id,c.c_name,max(s.s_score) '最高分',min(s.s_score) '最低分',avg(s.s_score) '平均分',
 (select count(s_score)/(select count(s_score) from score where c_id=c.c_id)
   from score where c.c_id=c_id and s_score>=60) '及格率',
(select count(s_score)/(select count(s_score) from score where c_id=c.c_id)
 from score where c_id=c.c_id and (s_score between 70 and 79) ) '中等率',
(select count(s_score)/(select count(s_score) from score where c_id=c.c_id)
  from score where c_id=c.c_id and (s_score between 80 and 89)) '良好率',
(select count(s_score)/(select count(s_score) from score where c_id=c.c_id)
  from score where c_id=c.c_id and (s_score between 90 and 100) ) '优秀率'
  from course c left join score s on c.c_id = s.c_id 
  group by c.c_id,c.c_name;


--# *19、按各科成绩进行排序，并显示排名(实现不完全)
--# mysql没有rank函数
--# 加@score是为了防止用union all 后打乱了顺序
select c1.s_id,c1.c_id,c1.c_name,@score:=c1.s_score,@i:=@i+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="01" order by sc.s_score desc) c1 ,
(select @i:=0) a
union all 
select c2.s_id,c2.c_id,c2.c_name,c2.s_score,@ii:=@ii+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="02" order by sc.s_score desc) c2 ,
(select @ii:=0) aa 
union all
select c3.s_id,c3.c_id,c3.c_name,c3.s_score,@iii:=@iii+1 from (select c.c_name,sc.* from course c 
left join score sc on sc.c_id=c.c_id
where c.c_id="03" order by sc.s_score desc) c3;
set @iii=0;
或
select a.* from (
  select st.s_id,st.s_name,c.c_name,s.s_score,count(distinct s2.s_score) '排名' 
  from score s join student st on s.s_id=st.s_id 
  join course c on s.c_id=c.c_id , score s2
  where c.c_id='01' and s2.s_score >= s.s_score and s2.c_id='01'
  group by st.s_id,st.s_name,c.c_name,s.s_score
  order by s.s_score desc
) a
union all 
select b.* from (
  select st.s_id,st.s_name,c.c_name,s.s_score,count(distinct s2.s_score) '排名' 
  from score s join student st on s.s_id=st.s_id 
  join course c on s.c_id=c.c_id , score s2
  where c.c_id='02' and s2.s_score >= s.s_score and s2.c_id='02'
  group by st.s_id,st.s_name,c.c_name,s.s_score
  order by s.s_score desc
) b
union all 
select c.* from (
  select st.s_id,st.s_name,c.c_name,s.s_score,count(distinct s2.s_score) '排名' 
  from score s join student st on s.s_id=st.s_id 
  join course c on s.c_id=c.c_id , score s2
  where c.c_id='03' and s2.s_score >= s.s_score and s2.c_id='03'
  group by st.s_id,st.s_name,c.c_name,s.s_score
  order by s.s_score desc
) c;

--20、查询学生的总成绩并进行排名
select st.s_id,st.s_name,(case when sum(sc.s_score) is null then 0 else sum(sc.s_score) end ) as '总成绩'
from student st left join score sc on st.s_id=sc.s_id 
group by st.s_id order by sum(sc.s_score) desc;
或
select st.s_id, st.s_name, (case when sum(s.s_score) is null then 0 else sum(s.s_score) end) '总成绩'
from student st left outer join score s on st.s_id = s.s_id
group by st.s_id, st.s_name
order by sum(s.s_score) desc;

--21、查询不同老师所教不同课程平均分从高到低显示 
select t.t_id,t.t_name,c.c_name,avg(sc.s_score) as '平均分' from teacher t
left join course c on (t.t_id=c.t_id)
left join score sc on (c.c_id=sc.c_id)
group by t.t_id 
order by avg(sc.s_score) desc;
或
select t.t_id, t.t_name, c.c_name, avg(s.s_score) '平均分'
from teacher t left outer join course c on t.t_id = c.t_id
left outer join score s on c.c_id = s.c_id
group by t.t_id, t.t_name, c.c_name
order by avg(s.s_score) desc; 


--22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id = s.s_id
left join course c on s.c_id = c.c_id
where s.c_id = '01' and (
s.s_score between
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '01' and s2.s_score >= s.s_score  and s2.c_id='01'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 3) 
and
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '01' and s2.s_score >= s.s_score  and s2.c_id='01'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 2)
)
union all
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id = s.s_id
left join course c on s.c_id = c.c_id
where s.c_id = '02' and (
s.s_score between
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '02' and s2.s_score >= s.s_score  and s2.c_id='02'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 3) 
and
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '02' and s2.s_score >= s.s_score  and s2.c_id='02'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 2)
)
union all
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id = s.s_id
left join course c on s.c_id = c.c_id
where s.c_id = '03' and (
s.s_score between
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '03' and s2.s_score >= s.s_score  and s2.c_id='03'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 3) 
and
(select distinct a.s_score
from (select s.s_score, count(distinct s2.s_score) '排名'
from score s inner join student st on s.s_id = st.s_id
inner join course c on s.c_id = c.c_id, score s2
where c.c_id = '03' and s2.s_score >= s.s_score  and s2.c_id='03'
group by st.s_id, st.s_name, c.c_name, s.s_score
order by s.s_score desc) a
where a.排名 = 2)
);


--*23、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 c.c_id,c.c_name,
(
   (select count(1) from score sc where sc.c_id=c.c_id and sc.s_score<=100 and sc.s_score>85)/
   (select count(1) from score sc where sc.c_id=c.c_id)
) as "100-85",
(
   (select count(1) from score sc where sc.c_id=c.c_id and sc.s_score>70 and sc.s_score<=85)/
   (select count(1) from score sc where sc.c_id=c.c_id)
) as "85-70",
(
   (select count(1) from score sc where sc.c_id=c.c_id and sc.s_score>60 and sc.s_score<=70)/
   (select count(1) from score sc where sc.c_id=c.c_id)
) as "70-60",
(
   (select count(1) from score sc where sc.c_id=c.c_id and sc.s_score>=0 and sc.s_score<=60)/
   (select count(1) from score sc where sc.c_id=c.c_id)
) as "60-0"
from course c order by c.c_id;
--count(1) 会统计表中的所有的记录数，包含字段为null 的记录。
--count(字段) 会统计该字段在表中出现的次数，忽略字段为null 的情况。即不统计字段为null 的记录。


--24、查询学生平均成绩及其名次 
set @i=0; --推荐
select a.*,@i:=@i+1 from(
 select st.s_id,st.s_name,round((case when avg(sc.s_score) is null then 0 else avg(sc.s_score) end) ,2) as '平均分'
 from student st left join score sc on st.s_id=sc.s_id 
 group by st.s_id order by sc.s_score desc
) as a 
或
select a.*, count(distinct b.平均成绩) '名次'
from
(select st.s_id, st.s_name, (case when avg(s.s_score) is null then 0 else avg(s.s_score) end) '平均成绩' 
from student st left join score s on st.s_id=s.s_id
group by st.s_id, st.s_name) a, 
(select st.s_id, (case when avg(s.s_score) is null then 0 else avg(s.s_score) end) '平均成绩' 
from student st left join score s on st.s_id=s.s_id
group by st.s_id) b
where a.平均成绩<=b.平均成绩
group by a.s_id
order by a.平均成绩 desc;

--25、查询各科成绩前三名的记录
select a.*
from (
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_id='01' and s.s_score between (
select distinct a1.s_score 
from (
select a2.s_id, a2.s_score, count(distinct a3.s_score) '名次'
from score a2, score a3
where a2.c_id='01' and a3.c_id='01' and a2.s_score<=a3.s_score
group by a2.s_id, a2.s_score
) a1
where a1.名次=3)
and
(select max(s_score) from score where c_id='01')
) a
union all    
select b.*
from (
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_id='02' and s.s_score between (
select distinct a1.s_score 
from (
select a2.s_id, a2.s_score, count(distinct a3.s_score) '名次'
from score a2, score a3
where a2.c_id='02' and a3.c_id='02' and a2.s_score<=a3.s_score
group by a2.s_id, a2.s_score
) a1
where a1.名次=3)
and
(select max(s_score) from score where c_id='02')
) b
union all    
select c.*
from (
select st.*, c.c_name, s.s_score
from student st 
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_id='03' and s.s_score between (
select distinct a1.s_score 
from (
select a2.s_id, a2.s_score, count(distinct a3.s_score) '名次'
from score a2, score a3
where a2.c_id='03' and a3.c_id='03' and a2.s_score<=a3.s_score
group by a2.s_id, a2.s_score
) a1
where a1.名次=3)
and
(select max(s_score) from score where c_id='03')
) c;


--26、查询每门课程被选修的学生数 
select c.c_id,c.c_name,count(s.s_id) as '选课人数'
from course c left join score s on c.c_id = s.c_id
group by c.c_id,c.c_name;


-- 27、查询出只有两门课程的全部学生的学号和姓名
select st.s_id,st.s_name from student st 
left join score sc on sc.s_id=st.s_id
inner join course c on c.c_id=sc.c_id 
group by st.s_id having count(1)=2
或
select st.s_id, st.s_name
from student st
left join score s on st.s_id=s.s_id
group by st.s_id, st.s_name
having count(s.s_id)=2;
或
select st.s_id,st.s_name 
from student st left join score s on st.s_id=s.s_id 
group by st.s_id,st.s_name having count(s.c_id)=2;


--28、查询男生、女生人数
select st.s_sex,count(1)
from student st group by st.s_sex;

--29、查询名字中含有"风"字的学生信息
select st.* from student st where st.s_name like "%风%";

--30、查询同名同性学生名单，并统计同名人数 
select st.*,count(1) from student st group by st.s_name,st.s_sex having count(1)>1

--31、查询1990年出生的学生名单
select st.* from student st where st.s_birth like "1990%";
select * from student where s_birth like "1990%";
或
select * from student where year(s_birth) = '1990';

--32、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 c.c_id,c.c_name,avg(sc.s_score)
from course c join score sc on  c.c_id = sc.c_id 
group by c.c_id,c.c_name order by avg(sc.s_score) desc,c.c_id;

--33、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 st.s_id,st.s_name,avg(sc.s_score) as "平均成绩" from 
student st left join score sc on st.s_id=sc.s_id
group by st.s_id having avg(sc.s_score)>=85;

--34、查询课程名称为"数学"，且分数低于60的学生姓名和分数 
select st.s_name,sc.s_score from 
student st join score sc on st.s_id=sc.s_id  and sc.s_score<60
join course c on sc.c_id=c.c_id and c.c_name="数学";
或
select st.s_name, s.s_score
from student st
left join score s on st.s_id=s.s_id
left join course c on c.c_id=s.c_id
where c.c_name='数学' and s.s_score<60;

--35、查询所有学生的课程及分数情况；
select st.s_id,st.s_name,c.c_name,sc.s_score from 
student st left join score sc on st.s_id=sc.s_id 
left join course c on sc.c_id=c.c_id 
order by st.s_id,c.c_name;

--36、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 st.s_id,st.s_name,c.c_name,sc.s_score from student st 
left join score sc on st.s_id = sc.s_id 
left join course c on sc.c_id=c.c_id 
where st.s_id in (
   select st2.s_id from student st2 
   left join score sc2 on st2.s_id=sc2.s_id 
   group by st2.s_id having min(sc2.s_score)>70
)
order by st.s_id;

--37、查询不及格的课程
select st.s_id,st.s_name,c.c_name,sc.s_score from student st
join score sc on st.s_id=sc.s_id and sc.s_score<60
join course c on c.c_id=sc.c_id
或者
select st.s_id,st.s_name,c.c_name,sc.s_score from 
student st left join score sc on st.s_id=sc.s_id
left join course c on sc.c_id=c.c_id 
where sc.s_score<60;

--38、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 st.s_id,st.s_name,sc.s_score from student st
join score sc on st.s_id=sc.s_id and sc.s_score>=80 and sc.c_id="01";

--39、求每门课程的学生人数
select c.c_id,c.c_name,count(1) from 
course c join score sc on c.c_id=sc.c_id
group by c.c_id;
或者
select c.c_id,c.c_name,count(st.s_id) from 
course c left join score sc on c.c_id=sc.c_id 
left join student st on sc.s_id=st.s_id 
group by c.c_id,c.c_name;

--40、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 st.*,c.c_name,sc.s_score,t.t_name 
from student st join score sc on st.s_id=sc.s_id
join course c on sc.c_id=c.c_id
join teacher t on c.t_id=t.t_id and t.t_name="张三"
order by sc.s_score desc limit 1;
或
select st.*,s.s_score,t.t_name,c.c_name
from student st left join score s on st.s_id = s.s_id
left join course c on s.c_id = c.c_id
left join teacher t on c.t_id=t.t_id
where t.t_name="张三" and s.s_score=(
   select max(s2.s_score) from score s2 
   left join course c2 on s2.c_id=c2.c_id
   left join teacher t2 on c2.t_id=t2.t_id 
   where t2.t_name="张三"
);

--41、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 st.s_id,st.s_name,sc.c_id,sc.s_score,c.c_name 
from student st left join score sc on st.s_id=sc.s_id
left join course c on sc.c_id=c.c_id 
where (
  select count(1) from student st2 
  left join score sc2 on st2.s_id=sc2.s_id 
  left join course c2 on sc2.c_id=c2.c_id 
  where sc.s_score=sc2.s_score and c.c_id!=c2.c_id
)>1

--42、查询每门功成绩最好的前两名 
select a.* from (
   select st.s_id,st.s_name,c.c_name,sc.s_score from student st
   left join score sc on st.s_id=sc.s_id 
   join course c on sc.c_id=c.c_id and c.c_id="01"
   order by sc.s_score desc limit 0,2
) as a
union all
select b.* from (
   select st.s_id,st.s_name,c.c_name,sc.s_score from student st
   left join score sc on st.s_id=sc.s_id 
   join course c on sc.c_id=c.c_id and c.c_id="02"
   order by sc.s_score desc limit 0,2
) as b
union all
select c.* from (
   select st.s_id,st.s_name,c.c_name,sc.s_score from student st
   left join score sc on st.s_id=sc.s_id 
   join course c on sc.c_id=c.c_id and c.c_id="03"
   order by sc.s_score desc limit 0,2
) as c;

--43、统计每门课程的学生选修人数（超过5人的课程才统计）。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.c_id,count(1) as "选修人数" from score sc left join course c on sc.c_id=c.c_id 
group by c.c_id having count(1)>5 order by count(1) desc,sc.c_id asc;
或
select c.c_id, count(s.s_id)
from course c left join score s on c.c_id=s.c_id
group by c.c_id
having count(s.s_id)>5
order by count(s.s_id) desc, c.c_id;

--44、检索至少选修两门课程的学生学号 
select st.s_id from student st left join score sc on st.s_id=sc.s_id 
group by st.s_id having count(1)>=2;

--45、查询选修了全部课程的学生信息
select st.* from student st left join score sc on st.s_id=sc.s_id 
group by st.s_id having count(sc.c_id)=(select count(*) from course);

--46、查询各学生的年龄(实岁)
select st.*,timestampdiff(year,st.s_birth,now()) from student st

--47、查询本周过生日的学生
select st.* from student st 
where week(now())=week(date_format(st.s_birth,'%Y%m%d'))

--48、查询下周过生日的学生(还要排除闰年情况)
select st.* from student st 
where week(now())+1=week(date_format(st.s_birth,'%Y%m%d'))

--49、查询本月过生日的学生
select st.* from student st 
where month(now())=month(date_format(st.s_birth,'%Y%m%d'))

--50、查询下月过生日的学生
-- 注意:当 当前月为12时,用month(now())+1为13而不是1,可用timestampadd()函数或mod取模
select st.* from student st 
where month(timestampadd(month,1,now()))=month(date_format(st.s_birth,'%Y%m%d'))
-- 或
select st.* from student st where (month(now()) + 1) mod 12 = month(date_format(st.s_birth,'%Y%m%d'))