数据库---表---多表查询

多表查询

• 多表连接查询
• 复合条件连接查询
• 子查询

一、准备表

#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)
;

#查看表结构和数据
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+

mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum(‘male‘,‘female‘) | NO | | male | |
| age | int(11) | YES | | NULL | |
| dep_id | int(11) | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+

mysql> select * from department;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+

mysql> select * from employee;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+

表department与employee

创建表，插入数据

二、多表连接查询

语法：

#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;

2.1交叉连接：不适用任何匹配条件，生成笛卡尔积

mysql> select * from employee,department;
        +----+------------+--------+------+--------+------+--------------+
        | id | name       | sex    | age  | dep_id | id   | name         |
        +----+------------+--------+------+--------+------+--------------+
        |  1 | egon       | male   |   18 |    200 |  200 | 技术         |
        |  1 | egon       | male   |   18 |    200 |  201 | 人力资源     |
        |  1 | egon       | male   |   18 |    200 |  202 | 销售         |
        |  1 | egon       | male   |   18 |    200 |  203 | 运营         |
        |  2 | alex       | female |   48 |    201 |  200 | 技术         |
        |  2 | alex       | female |   48 |    201 |  201 | 人力资源     |
        |  2 | alex       | female |   48 |    201 |  202 | 销售         |
        |  2 | alex       | female |   48 |    201 |  203 | 运营         |
        |  3 | wupeiqi    | male   |   38 |    201 |  200 | 技术         |
        |  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
        |  3 | wupeiqi    | male   |   38 |    201 |  202 | 销售         |
        |  3 | wupeiqi    | male   |   38 |    201 |  203 | 运营         |
        |  4 | yuanhao    | female |   28 |    202 |  200 | 技术         |
        |  4 | yuanhao    | female |   28 |    202 |  201 | 人力资源     |
        |  4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
        |  4 | yuanhao    | female |   28 |    202 |  203 | 运营         |
        |  5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
        |  5 | liwenzhou  | male   |   18 |    200 |  201 | 人力资源     |
        |  5 | liwenzhou  | male   |   18 |    200 |  202 | 销售         |
        |  5 | liwenzhou  | male   |   18 |    200 |  203 | 运营         |
        |  6 | jingliyang | female |   18 |    204 |  200 | 技术         |
        |  6 | jingliyang | female |   18 |    204 |  201 | 人力资源     |
        |  6 | jingliyang | female |   18 |    204 |  202 | 销售         |
        |  6 | jingliyang | female |   18 |    204 |  203 | 运营         |
        +----+------------+--------+------+--------+------+--------------+  

交叉连接

不适用任何匹配条件。生成笛卡尔积

2.2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.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;
        +----+-----------+------+--------+--------------+
        | id | name      | age  | sex    | name         |
        +----+-----------+------+--------+--------------+
        |  1 | egon      |   18 | male   | 技术         |
        |  2 | alex      |   48 | female | 人力资源     |
        |  3 | wupeiqi   |   38 | male   | 人力资源     |
        |  4 | yuanhao   |   28 | female | 销售         |
        |  5 | liwenzhou |   18 | male   | 技术         |
        +----+-----------+------+--------+--------------+

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

内连接

内连接

2.3外连接之左连接：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.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
+----+------------+--------------+
| id | name       | depart_name  |
+----+------------+--------------+
|  1 | egon       | 技术         |
|  5 | liwenzhou  | 技术         |
|  2 | alex       | 人力资源     |
|  3 | wupeiqi    | 人力资源     |
|  4 | yuanhao    | 销售         |
|  6 | jingliyang | NULL         |
+----+------------+--------------+

左连接

2.4

原文地址：https://www.cnblogs.com/Mryang123/p/9005178.html