day 7-17 多表查询

一. 准备表

#部门表
create table dep(
id int,
name varchar(20) 
);
#员工表
create table emp(
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 employee(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)
;

查看表结构

二. 多表连接查询

#语法:
select * from 表1 <inner,left,right> join 表2 on 表1.字段=表2.字段;

　　1.交叉连接:没有任何查询条件,直接生成笛卡尔积.(笛卡尔积:https://baike.baidu.com/item/%E7%AC%9B%E5%8D%A1%E5%B0%94%E4%B9%98%E7%A7%AF)

　　select * from employee,department;

　　

　　2.内连接:找出两张表共有的部分,相当于利用条件在笛卡尔积的基础上进行筛选正确的结果.(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 employee inner join department on employee.dep_id = department.id;

#还可以使用where条件来查询,建议使用上面的语句
select * from employee,department where employee.dep_id = department.id;

　　3.左连接:优先显示坐标表的全部记录　　

select * from employee left join department on employee.dep_id = department.id;

select * from department left join employee on employee.dep_id = department.id;

　　4. 右连接:优先显示右边的全部记录.　　

#优先显示员工表(employee)的信息
select * from department right join employee on employee.dep_id = department.id;　　
#优先显示部门表(department)的信息
select * from employee right join department on employee.dep_id = department.id;

　　5.全外连接:显示2张表中的所有记录

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

三. 符合条件连接查询

　　1.需求,查询所有年龄大于25岁的员工姓名,年龄和部门名称.　　

select employee.name,employee.age,department.name from employee inner join department on employee.dep_id = department.id where age >25;

　　2.按照员工的年龄,进行降序排序　　

select * from  employee inner join department on employee.dep_id = department.id  order by age desc;