一、准备

建表与数据准备

sql 示例：

#建表
create table department(
id int,
name varchar(20) 
);
create table employee(
id int primary key auto_increment,
name varchar(20),
gende 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,gender,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)
;
#查看表结构和数据
mysql> desc department;

mysql> desc employee_1;

mysql> select * from department;

mysql> select * from employee_1;

表department与employee

二、多表连接查询

#重点：外链接语法
SELECT 字段列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.字段 = 表2.字段;

1 交叉连接：不适用任何匹配条件。生成笛卡尔积

mysql> select * from employee_1,department;

2 内连接：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
mysql> select employee_1.id,employee_1.name,employee_1.gender,employee_1.age,department.name from employee_1 inner join department on employee_1.dep_id=department.id;

#上述sql等同于
mysql> select employee_1.id,employee_1.name,employee_1.gender,employee_1.age,department.name from employee_1,department where employee_1.dep_id=department.id;

3 外链接之左连接：优先显示左表全部记录

#以左表为准，即找出所有员工信息，当然包括没有部门的员工
#本质就是：在内连接的基础上增加左边有右边没有的结果
mysql> select employee_1.id,employee_1.name,employee_1.gender,employee_1.age,department.name from employee_1 left join department on employee_1.dep_id=department.id;

4 外链接之右连接：优先显示右表全部记录

#以右表为准，即找出所有部门信息，包括没有员工的部门
#本质就是：在内连接的基础上增加右边有左边没有的结果
mysql> select employee_1.id,employee_1.name,employee_1.gender,employee_1.age,department.name from employee_1 right join department on employee_1.dep_id=department.id;

5 全外连接：显示左右两个表全部记录

全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
#注意：mysql不支持全外连接 full JOIN
#强调：mysql可以使用此种方式间接实现全外连接
mysql> select * from employee_1 left join department on employee_1.dep_id=department.id
    -> union
    -> select * from employee_1 right join department on employee_1.dep_id=department.id
    -> ;
#查看结果

#注意 union与union all的区别：union会去掉相同的纪录

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,即找出年龄大于25岁的员工以及员工所在的部门
mysql> select employee_1.id,employee_1.name,employee_1.age,department.name from employee_1 inner join department on employee_1.dep_id=department.id where employee_1.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
mysql> select * from employee_1,department where employee_1.dep_id=department.id and age>25 order by age asc;