MySQL 多表查询 学习与练习

#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)
;

#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;

#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;

#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;

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; 

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

and age>25;

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

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

#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

#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);
);

(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>

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