多表查询

一、介绍

首先先准备表

员工表和部门表

#建表
create table department(
id int,
name varchar(20) 
);

create table employee1(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into employee1(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;

查看表：

二、多表连接查询

1.交叉连接：不适用任何匹配条件。生成笛卡尔积、

select * from employee1 ,department;

2.内连接：找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果。（只连接匹配的行）

#找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果
#department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来
select * from employee1,department where employee1.dep_id=department.id;

#上面用where表示的可以用下面的内连接表示，建议使用下面的那种方法
select * from employee1 inner join department on employee1.dep_id=department.id;

#也可以这样表示哈

select employee1.id,employee1.name,employee1.age,employee1.sex,department.name from 
employee1,department where employee1.dep_id=department.id;

 

注意：内连接的join可以忽略不写，但是还是加上看起来清楚点

3.左连接：优先显示左表全部记录。

#左链接：在按照on的条件取到两张表共同部分的基础上，保留左表的记录
select * from employee1 left join department on department.id=employee1.dep_id;
select * from department left join  employee1 on department.id=employee1.dep_id;

 

4.右链接：优先显示右表全部记录。

#右链接：在按照on的条件取到两张表共同部分的基础上，保留右表的记录
select * from employee1 right join department on department.id=employee1.dep_id;

select * from department right join employee1 on department.id=employee1.dep_id;

5.全外连接：显示左右两个表的全部记录。

注意：mysql不支持全外连接 full join
强调：mysql可以使用union间接实现全外连接

select * from employee1 left join department on department.id=employee1.dep_id
union
select * from employee1 right join department on department.id=employee1.dep_id; 

三、符合条件连接查询

示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,
即找出公司所有部门中年龄大于25岁的员工

select * from employee1 inner join department on employee1.dep_id=department.id 

and age>25;

 

示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示

select * from employee1 inner join department on employee1.dep_id=department.id 
=

and age>25 and age>25 order by age asc;

四、子查询

#1：子查询是将一个查询语句嵌套在另一个查询语句中。
#2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。
#3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4：还可以包含比较运算符：= 、 !=、> 、<等

小练习

#查询平均年龄在25岁以上的部门名
select name from department where id in (
select dep_id from employee group by dep_id having avg(age) > 25
);
#查看技术部员工姓名
select name from employee where dep_id = (select id from department where name='技术');

#查看小于2人的部门名
select name from department where id in (
select dep_id from employee1 group by dep_id having count(id) < 2
)
union
select name from department where id not in (select distinct dep_id from employee1);

#提取空部门                              #有人的部门
select * from department where id not in (select distinct dep_id from employee1);


或者：
select name from department where id in
(
select dep_id from employee1 group by dep_id having count(id) < 2
union
select id from department where id not in (select distinct dep_id from employee1);
);

五、综合练习

一、SELECT语句关键字的定义顺序

SELECT DISTINCT <select_list>
FROM <left_table>
<join_type> JOIN <right_table>
ON <join_condition>
WHERE <where_condition>
GROUP BY <group_by_list>
HAVING <having_condition>
ORDER BY <order_by_condition>
LIMIT <limit_number>

二 、SELECT语句关键字的执行顺序

(7)     SELECT 
(8)     DISTINCT <select_list>
(1)     FROM <left_table>
(3)     <join_type> JOIN <right_table>
(2)     ON <join_condition>
(4)     WHERE <where_condition>
(5)     GROUP BY <group_by_list>
(6)     HAVING <having_condition>
(9)     ORDER BY <order_by_condition>
(10)    LIMIT <limit_number>

init.sql文件内容

 从init.sql文件中导入数据

#准备表、记录
mysql> create database db1;
mysql> use db1;
mysql> source /root/init.sql

1、查询所有的课程的名称以及对应的任课老师姓名

2、查询学生表中男女生各有多少人

3、查询物理成绩等于100的学生的姓名

4、查询平均成绩大于八十分的同学的姓名和平均成绩

5、查询所有学生的学号，姓名，选课数，总成绩

6、 查询姓李老师的个数

7、 查询没有报李平老师课的学生姓名

8、 查询物理课程比生物课程高的学生的学号

9、 查询没有同时选修物理课程和体育课程的学生姓名

10、查询挂科超过两门(包括两门)的学生姓名和班级
、查询选修了所有课程的学生姓名

12、查询李平老师教的课程的所有成绩记录
 
13、查询全部学生都选修了的课程号和课程名

14、查询每门课程被选修的次数

15、查询之选修了一门课程的学生姓名和学号

16、查询所有学生考出的成绩并按从高到低排序（成绩去重）

17、查询平均成绩大于85的学生姓名和平均成绩

18、查询生物成绩不及格的学生姓名和对应生物分数

19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名

20、查询每门课程成绩最好的前两名学生姓名

21、查询不同课程但成绩相同的学号，课程号，成绩

22、查询没学过“叶平”老师课程的学生姓名以及选修的课程名称；

23、查询所有选修了学号为1的同学选修过的一门或者多门课程的同学学号和姓名；

24、任课最多的老师中学生单科成绩最高的学生姓名

答案待续。。。。

1、查询所有的课程的名称以及对应的任课老师姓名
select cname 课程名称,tname 老师姓名 from course  left join teacher on course.teacher_id = teacher.tid;

2、查询学生表中男女生各有多少人
select gender ,count(gender) from student group by gender;

3、查询物理成绩等于100的学生的姓名
select sname from student inner join score on student.sid = score.student_id
join course on score.course_id = course.cid
where cname = '物理' and num = 100;

4、查询平均成绩大于八十分的同学的姓名和平均成绩
(1):
select sname 姓名,avg(num) 平均成绩 from student inner join score on student.sid = score.student_id 
group by student_id having avg(num)>80;
(2):
select sname,avg(num) from student inner join score on student.sid = score.student_id 
group by student_id having avg(num)>80

5、查询所有学生的学号，姓名，选课数，总成绩
select student.sid 学号,sname 姓名,count(course_id) 选课数,sum(num) 总成绩 from student left join score on student.sid = score.student_id
group by student.sid ;
6、 查询姓李老师的个数
select count(tid) from teacher where tname like '李%';

7、 查询没有报李平老师课的学生姓名
select sname 姓名 from student where sid not in (
select student_id from score where course_id in(
select cid from course where teacher_id =(
select tid from teacher where tname = '李平老师'
)));

8、 查询物理课程比生物课程高的学生的学号
select t1.student_id  学号 from 
(select student_id ,num from score inner join course on score.course_id=course.cid  where cname='物理' )as t1
inner join 
(select student_id , num from score inner join course on score.course_id=course.cid  where cname = '生物') as t2
on t1.student_id = t2.student_id
where t1.num>t2.num;



9、 查询没有同时选修物理课程和体育课程的学生姓名
(1):
select sname from student inner join score on student.sid = score.student_id
join course on course.cid=score.course_id and cname in ('物理','体育') 
group by student_id having count(course_id)!=2;

(2):
select sname from student join score  on student.sid = score.student_id
join course on course.cid=score.course_id where cname ='物理' or cname= '体育'
group by student_id having count(course_id) !=2;

10、查询挂科超过两门(包括两门)的学生姓名和班级
select sname 姓名,caption 班级 from student inner join score on student.sid = score.course_id
join class on class.cid = score.course_id
where num<60 group by student_id having count(course_id)>=2;

11、查询选修了所有课程的学生姓名
(1)
select sname 姓名 ,所有的课程数 from student inner join 
(select student_id,count(course_id) 所有的课程数 from score group by student_id having count(course_id) = (
select count(cid) from course)) as t1
on t1.student_id = student.sid;

(2)
select sname,count(course_id) from student inner join score on student.sid = score.student_id
group by student_id having count(course_id)=(select count(cid) from course);

12、查询李平老师教的课程的所有成绩记录
(1):
select num from score inner join course on course.cid=score.course_id
join teacher on teacher.tid=course.teacher_id
where tname = '李平老师';

(2):
select num from score where course_id in(select cid from course where teacher_id=
(select tid from teacher where tname='李平老师'));

13、查询全部学生都选修了的课程号和课程名
select cid 课程号,cname 课程名 from course
select ;
14、查询每门课程被选修的次数 
(1)
select course.cname,count(student_id) 选课人数 from score inner join course on score.course_id=course.cid
group by course_id;

(2):也可以按照name分组
select course.cname,count(student_id) 选课人数 from score inner join course on score.course_id=course.cid
group by cname;

15、查询之选修了一门课程的学生姓名和学号
select sname 姓名,student_id 学号 from student inner join score on student.sid = score.student_id
group by student_id having count(course_id)=1;

16、查询所有学生考出的成绩并按从高到低排序（成绩去重）
select distinct num from score order by num desc;

17、查询平均成绩大于85的学生姓名和平均成绩
(1):
select sname 姓名,avg(num) 平均成绩 from student inner join score on student.sid = score.student_id
group by student_id having avg(num)>85;

(2):
select student.sname,avg_num from student inner join
(select student_id,avg(num) as avg_num from score group by student_id having avg(num) > 85
) t1
on student.sid=t1.student_id;


18、查询生物成绩不及格的学生姓名和对应生物分数
(1):
select student.sname ,num 生物成绩 from student inner join score on student.sid = score.student_id
join course on course.cid=score.course_id
where cname='生物' and num<60;

(2):
select student.sname,t1.num from student inner join
(
select student_id,num from score
where course_id=(select cid from course where cname='生物') and num < 60
) t1
on t1.student_id=student.sid
;

19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名
select sname from student where sid=(
select student_id from score where course_id in (
select cid from course where teacher_id=(select tid from teacher where tname='李平老师')
) group by student_id order by avg(num) desc limit 1
)