为什么需要多表查询：

因为我们不可能把所有数据都放在一张表里

我们把不同数据存储 放在一张一张不同表 方便管理，但我们为了方便管理，把数据拆分到一张一张表去存储。

但是数据还是一个整体，数据之间是有关联关系 ，那就要把分散的数据，合并到一起进行查询

多表查询概念：

连表操作

通过连接方式，把有关系的表拼成一个整体，进行关联查询。

就是把多张表记录 合并到一张表去查询

多表查询准备 数据库 数据表

新建数据库

create database db6 charset=utf8;

use db6;

**#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
('mike','male',18,200),
('alex','female',48,201),
('jack','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    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (1.30 sec)

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    |                |
+--------+-----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> select * from employee;
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  1 | mike       | male   |   18 |    200 |
|  2 | alex       | female |   48 |    201 |
|  3 | jack       | male   |   38 |    201 |
|  4 | yuanhao    | female |   28 |    202 |
|  5 | liwenzhou  | male   |   18 |    200 |
|  6 | jingliyang | female |   18 |    204 |
+----+------------+--------+------+--------+
6 rows in set (0.32 sec)

mysql> select * from department;
+------+--------------+
| id   | name         |
+------+--------------+
|  200 | 技术         |
|  201 | 人力资源     |
|  202 | 销售         |
|  203 | 运营         |
+------+--------------+
4 rows in set (0.00 sec)