多表查询

#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)
;

查询

1、select * from employee1 ,department;

　　生成一个N*M的表

2、按相同部分查询

#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
#只显示符合条件的记录
select * from employee1,department where employee1.dep_id=department.id;
==========================
select * from employee1 inner join department on employee1.dep_id=department.id;

join可以忽略不写

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; 

 

子查询：

#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);
);