mysql 查询的练习与提高

-- 创建数据库school
create database school;

-- 选择进入school数据库
use school;


-- ------------建表导数-------------
-- 创建stu
create table stu(
s_id varchar(10) primary key,
s_name varchar(10) not null,
s_birth date,
s_sex varchar(10));

-- 导入数据
insert into stu values
('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' , '1992-04-21' , '女'),
('08' , '王菊' , '1990-01-20' , '女');

select * from stu; -- 检查数据
select count(*) from stu; -- 检查总行数


-- 创建co
create table co(
c_id varchar(10) primary key,
c_name varchar(10),
t_id varchar(10));

-- 导入数据
insert into co values
('01' , '语文' , '02'),
('02' , '数学' , '01'),
('03' , '英语' , '03');

select * from co; -- 检查数据
select count(*) from co; -- 检查总行数


-- 创建te
create table te(
t_id varchar(10) primary key,
t_name varchar(10));

-- 导入数据
insert into te values
('01' , '张三'),
('02' , '李四'),
('03' , '王五');

select * from te; -- 检查数据
select count(*) from te; -- 检查总行数


-- 创建sc
create table sc(
s_id varchar(10),
c_id varchar(10),
score int);

-- 导入数据
insert into sc values
('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);

select * from sc; -- 检查数据
select count(*) from sc; -- 检查总行数

　　练习题

-- ----------------查询提高------------------
-- 1、查询考试成绩不及格的学生信息
select s_id 
from sc
where score <60;

select *
from stu
where s_id in 
(select s_id 
from sc
where score <60);

select distinct stu.*
from sc left join stu on stu.s_id=sc.s_id
where score < 60;

#笛卡儿积  不能有非匹配项
select distinct stu.*
from stu , sc
where stu.s_id = sc.s_id and score <60;



-- 2、查询姓‘李’的老师的数量   ##模糊查询
select count(t_id)
from te
where t_name like '李%';


-- 3、查询没有出现过考试成绩不及格的学生：s_id,c_name,score
##1、考试成绩大于60 的学生
select distinct s_id 
from  sc
where score<60;

select stu.s_id,stu.s_name,co.c_name,score
from stu left join sc on stu.s_id = sc.s_id left join co on sc.c_id = co.c_id
where stu.s_id not in (
select distinct s_id 
from  sc
where score<60);


-- 4、查询平均成绩大于70分的课程
#第一种方法
select c_name, avg(score) ,co.c_id
from sc left join co on sc.c_id = co.c_id
group by sc.c_id
having avg(score)>70;

#第二种方法

select c_id 
from sc
group by c_id
having avg(score)>70;

select c_name ,c_id
from co 
where c_id in
(
select c_id 
from sc
group by c_id
having avg(score)>70);

-- 5、查询平均成绩大于60分的学生:s_id,s_name,平均成绩
select stu.s_id , s_name , avg(score)
from sc left join stu on stu.s_id = sc.s_id
group by sc.s_id
having avg(score)>60;

##第二种方法
select s_id,avg(score)
from sc
group by s_id
having avg(score)>60; 

select s_id , s_name 
from stu
where s_id in
(select s_id
from sc
group by s_id
having avg(score)>60);
-- 6、查询至少选修了两门课程的学生：s_id,s_name,选课门数
select sc.s_id,s_name,count(sc.s_id)
from sc left join stu on sc.s_id = stu.s_id
group by sc.s_id
having count(sc.s_id) >=2;

##第二种方法
select s_id ,count(c_id)
from sc 
group by s_id
having count(c_id) >= 2;

select s.*,s_name
from (select s_id ,count(c_id)
from sc 
group by s_id
having count(c_id) >= 2) s left join stu on s.s_id=stu.s_id;

-- 7、查询选修了全部课程的学生信息   ##思路：先查询出总共的课程门数有多少，然后连接学生表和成绩表，通过学生id分组，当成绩表中的选修门数等于所有的课程数时输出
#查找课程门数 
 
select count(c_id) from co;

select stu.*,count(sc.c_id) 选修门数
from sc left join stu on sc.s_id = stu.s_id
group by s_id
having 选修门数=(select count(c_id) from co);



-- 8、查询每门课程被选修的学生数   ##课程表中的
select co.c_id,c_name,count(*) 选修人数
from co left join sc on co.c_id = sc.c_id
group by co.c_id;





-- 9、查询90后学生的男女人数
##第一种方法
select sum(datediff(s_birth,'1989-12-12')>0) 90后人数,s_sex   #90后的人数
from stu
group by s_sex;

##第二种方法
select s_sex,count(s_id) 人数
from stu
where year(s_birth)>=1990
group by s_sex;

-- 10、查询至少有3门课程及格的学生


select sc.s_id ,s_name,count(score>60) 
from sc left join stu on sc.s_id = stu.s_id
where score >=60
group by sc.s_id
having count(score>60)>=3;


##查询每位同学的及格门数
select s_id , count(c_id)  及格门数
from sc
where score >= 60
group by s_id
having 及格门数>=3;

#子查询
select s.s_id,s_name ,及格门数
from (
select s_id , count(c_id)  及格门数
from sc
where score >= 60
group by s_id
having 及格门数>=3) s 
left join stu on s.s_id = stu.s_id;

-- 11、查询所有学生的s_id,s_name,选课门数,所选课程总成绩
select stu.s_id,s_name,count(c_id),sum(score)
from stu left join sc on stu.s_id = sc.s_id
group by stu.s_id;


-- 12、查询至少有两门成绩不及格的学生：s_id,s_name,不及格门数,平均成绩
#第一种方法
select sc.s_id,s_name,count(score<60),avg(score)
from sc left join stu on sc.s_id= stu.s_id
where score <60
group by sc.s_id
having count(score<60)>=2;
#第二种方法
select sc.s_id,s_name,count(c_id) 不及格门数,avg(score)
from sc left join stu on stu.s_id= sc.s_id
group by sc.s_id
having sum(score<60)>=2;

select sum(score <60) from sc;
-- 13、学过"张三"老师课程的学生信息   #四表连接顺序， 教师，课程，成绩，学生

#方法一
select *
from te left join co on te.t_id = co.t_id 
left join sc on co.c_id = sc.c_id 
left join stu on sc.s_id = stu.s_id
where t_name = '张三';
 
 #方法二  子查询
select *
from stu
where s_id in
(select s_id
from te left join co on te.t_id = co.t_id left join sc on co.c_id = sc.c_id
where t_name = '张三');


-- 14、查询所有学生不同课程的成绩及平均成绩：s_id,语文,数学,英语,平均成绩
#第一种方法
select s_id,sum(case when c_id='01' then score else 0 end) 语文,
			sum(case when c_id='02' then score else 0 end) 数学,
			sum(case when c_id='03' then score else 0 end) 英语,
			avg(score) 平均成绩
from sc
group by s_id;


select stu.s_id,sum(case when c_id='01' then score else 0 end) 语文,
			sum(case when c_id='02' then score else 0 end) 数学,
			sum(case when c_id='03' then score else 0 end) 英语,
			avg(score) 平均成绩
from stu left join sc on stu.s_id=sc.s_id
group by stu.s_id;

##第二种方法
select s_id,sum(if( c_id='01',score ,0)) 语文,
			sum(if( c_id='02',score ,0)) 数学,
			sum(if( c_id='03',score ,0)) 英语,
			avg(score) 平均成绩
from sc
group by s_id;
-- 15、查询不同老师所授不同课程的平均成绩
select t_name,avg(score),c_name
from te left join co on te.t_id = co.t_id left join sc on co.c_id = sc.c_id
group by te.t_id,co.c_id;



-- 16、查询和01号同学所学课程完全一样的学生信息
select group_concat(c_id) from sc group by s_id having s_id = '01'; #找出01所选修的课程


select c_id
from sc
where s_id='01';

select stu.s_id,group_concat(c_id order by c_id) 选修课程
from stu left join sc on sc.s_id= stu.s_id
group by stu.s_id
having 选修课程 = (select group_concat(c_id order by c_id) from sc group by s_id having s_id = '01');



-- 17、查询至少有一门课与01号同学所学课程一样的学生信息
select c_id
from sc
where s_id='01';

select distinct stu.*
from stu left join sc on sc.s_id= stu.s_id
where c_id in  (select c_id
from sc
where s_id='01');




-- 18、查询选修了01号课程并且也选修了02号课程的学生信息

##第一种方法
select s_id,count(c_id)
from sc
where c_id in (01,02)
group by s_id
having count(c_id)=2;


select stu.*
from sc left join stu on sc.s_id= stu.s_id
where c_id in (01,02)
group by sc.s_id
having count(c_id)=2;

##第二种方法
select distinct stu.*
from sc left join stu on sc.s_id= stu.s_id
where stu.s_id in (select s_id from sc where c_id='01') and 
		stu.s_id in(select s_id from sc where c_id='02');

##第三种方法
select stu.*,group_concat(c_id order by c_id) 选修课程
from sc left join stu on sc.s_id= stu.s_id
group by stu.s_id
having 选修课程 like '01,02%';

#第四种方法
select stu.*
from sc left join stu on sc.s_id= stu.s_id
group by stu.s_id
having sum(c_id=01)+sum(c_id=02)=2;
having sum(c_id =(01,02))=2;
-- 19、查询01号课程比02号课程分数高的学生信息及成绩
#第一步
select s_id,score from sc where c_id = '01';
select s_id,score from sc where c_id = '02';

#第二步
select *
from stu left join (select s_id,score from sc where c_id = '01') s1 on stu.s_id=s1.s_id left join
(select s_id,score from sc where c_id = '02') s2 on stu.s_id=s2.s_id
where s1.score>s2.score;


-- 20、查询选修了01号课程但是没有选修02号课程的学生信息
select distinct stu.*
from sc left join stu on sc.s_id= stu.s_id
where stu.s_id in (select s_id from sc where c_id='01') and 
		stu.s_id not in(select s_id from sc where c_id='02');

#第二种方法
select stu.*
from sc left join stu on sc.s_id= stu.s_id
group by stu.s_id
having sum(c_id=01)=1 and sum(c_id=02)=0;









-- 21、查询选修'张三'老师所授课程的学生中，分数最高的学生信息及成绩
select stu.*,score
from te left join co on te.t_id = co.t_id 
left join sc on co.c_id = sc.c_id 
left join stu on sc.s_id = stu.s_id
where t_name = '张三'
order by score desc
limit 1;

select stu.*,max(score)
from te left join co on te.t_id = co.t_id 
left join sc on co.c_id = sc.c_id 
left join stu on sc.s_id = stu.s_id
where t_name = '张三';


-- 22、查询各科成绩最高分,最低分,平均分,合格率:cname
select c_name ,max(score),min(score),avg(score),sum(score>=60)/count(co.c_id) 合格率
from co left join sc on sc.c_id = co.c_id
group by co.c_id;

##考试成绩合格的人数
select sum(score >=60) from sc;
-- 23、统计各科成绩各分数段人数
select co.c_id ,c_name,
sum(score >=85) '[100,85]',
sum(score >=70 and score <85) '[70,85]',
sum(score >=60 and score <70 ) '[60,70]',
sum( score <60 ) '[0,60]'
from co left join sc on co.c_id = sc.c_id
group by co.c_id;


select c_id , sum(case when score >=85 then 1 else 0 end) '[100,85]',
sum(case when score between 70 and 85 then 1 else 0 end) '[70,85]',
sum(case when score between 60 and 70 then 1 else 0 end) '[60,70]',
sum(case when score <60 then 1 else 0 end) '[0,60]'
from sc
group by c_id;



-- 24、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 *
from sc
group by s_id
having max(score)<70;

select s_name,c_name,score
from stu 
left join sc on sc.s_id= stu.s_id 
left join co on sc.c_id = co.c_id
	where stu.s_id not in (
	select s_id
	from sc
	group by s_id
	having max(score)<70);



/*
select s_name,c_name,score
from sc left join stu on sc.s_id= stu.s_id left join co on sc.c_id = co.c_id
where score>70
group by score;*/
-- 25、按各科成绩进行排序，并显示排名；s_id,c_id,score,排名

use school;
set @ss:=0;

select a.*,@ss:=@ss+1 as 排名
from (select c_id,s_id,score
from sc
group by c_id,s_id
order by c_id,score desc) a;

select c_id,s_id,score
from sc
group by c_id,s_id
order by c_id,score desc;




select @ss;