10 多表查询--连表操作

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

create table employee(
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)
;

内连接：

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

左连接：在内连接的基础上保留左表的记录

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

右连接：

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

全外连接：在内连接的基础上保留左右两表没有对应关系的记录

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;

#查询平均年龄大于30岁的部门

select department.name,avg(age) from employee inner join department on employee.dep_id=department.id
group by department.name
having avg(age)>30;

SELECT 语句关键字的定义顺序

SELECT 语句关键字的执行顺序

执行FROM

执行ON

执行外部行