為什麼需要多表查詢：

因為不可以把所有資料都放在一張表裡

我們把不同資料儲存 放在一張一張不同表 方便管理

但是資料還是一個整體，資料之間是有關聯關係 那就要把分散的資料，合併到一起進行查詢

 多表查詢概念：

通過連線方式，把有關係的表拼成一個整體，進行關聯查詢。

就是把多張表記錄 合併到一張表去查詢

新建資料庫

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
);

#插入資料

insert into department values
(200,'技術'),
(201,'人力資源'),
(202,'銷售'),
(203,'運營');

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)
;

#查看錶結構和資料

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 | 技術         |
|  201 | 人力資源     |
|  202 | 銷售         |
|  203 | 運營         |
+------+--------------+
4 rows in set (0.00 sec)