数据库_多表查询_内连接&外连接

首先明确一下定义:

内连接:在多表查询的时候,只检索出表之间相互匹配的行(满足匹配条件的行),结果中不包括两个表之间不匹配的行。

一个不太准确但易懂的例子:比如有两个表,表1存储了 (1,2,3,4,5 )这五个数字。表2存储了(3,4,5,6,7,8)这6个数字。多表查询时两个表的关联条件是 “表1 = 表2”.

则检索的输出是两个表的交集。也就是说,3,4,4,5。

[En]

Then the output of the retrieval is the intersection of the two tables. That is, 3, 4, 4, 5.

数据库_多表查询_内连接&外连接

外连接:在多表查询的时候,处理检索出内连接的数据外,还返回左表(或者右表)中不满足匹配条件的行。这种连接称为左(或者右)外连接。

也是上面的小例子:左侧外部链接的返回结果为(1),右侧外部链接的返回结果为(3),右侧外部连接的返回结果为(3)。

[En]

It is also the small example above: the return result of the left outer link is (1), the return result of the right outer link is (3), and the return result of the right outer connection is (3).

如果是 左外连接,则连接条件中左边的表也称为 主表 ,右边的表称为 从表 。如下简图:

数据库_多表查询_内连接&外连接

如果是 右外连接,则连接条件中右边的表也称为 主表 ,左边的表称为 从表

数据库_多表查询_内连接&外连接

代码实现:

以下是代码示例中使用的三个表

[En]

Here are three tables to be used in the code example

employees表:

+----------------+-------------+------+-----+---------+-------+
| Field          | Type        | Null | Key | Default | Extra |
+----------------+-------------+------+-----+---------+-------+
| employee_id    | int         | NO   | PRI | 0       |       |
| first_name     | varchar(20) | YES  |     | NULL    |       |
| last_name      | varchar(25) | NO   |     | NULL    |       |
| email          | varchar(25) | NO   | UNI | NULL    |       |
| phone_number   | varchar(20) | YES  |     | NULL    |       |
| hire_date      | date        | NO   |     | NULL    |       |
| job_id         | varchar(10) | NO   | MUL | NULL    |       |
| salary         | double(8,2) | YES  |     | NULL    |       |
| commission_pct | double(2,2) | YES  |     | NULL    |       |
| manager_id     | int         | YES  | MUL | NULL    |       |
| department_id  | int         | YES  | MUL | NULL    |       |
+----------------+-------------+------+-----+---------+-------+

departments表:

+-----------------+-------------+------+-----+---------+-------+
| Field           | Type        | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------+-------+
| department_id   | int         | NO   | PRI | 0       |       |
| department_name | varchar(30) | NO   |     | NULL    |       |
| manager_id      | int         | YES  | MUL | NULL    |       |
| location_id     | int         | YES  | MUL | NULL    |       |
+-----------------+-------------+------+-----+---------+-------+

locations表:

+----------------+-------------+------+-----+---------+-------+
| Field          | Type        | Null | Key | Default | Extra |
+----------------+-------------+------+-----+---------+-------+
| location_id    | int         | NO   | PRI | 0       |       |
| street_address | varchar(40) | YES  |     | NULL    |       |
| postal_code    | varchar(12) | YES  |     | NULL    |       |
| city           | varchar(30) | NO   |     | NULL    |       |
| state_province | varchar(25) | YES  |     | NULL    |       |
| country_id     | char(2)     | YES  | MUL | NULL    |       |
+----------------+-------------+------+-----+---------+-------+

注:以下所用语法:JOIN…ON 为 SQL99 中的多表查询实现语法。

内连接:检索返回员工姓名,部门名组成的表(需要用到employees表及departments表)

mysql> SELECT last_name,department_name
    -> FROM employees e  # 表1 JOIN 表2 ON 匹配条件;
    -> JOIN departments d
    -> ON e.department_id = d.department_id;   # 关联条件

检索一个返回员工姓名、部门名称和部门所在城市的表。

[En]

Retrieve a table that returns the name of the employee, the name of the department, and the city where the department is located.

mysql> SELECT last_name,department_name,city
    -> FROM employees e
    -> JOIN departments d
    -> ON e.department_id = d.department_id
    -> JOIN locations l
    -> ON d.location_id = l.location_id;
    /*
    表1
    JOIN 表2
    ON 表1和表2的匹配条件
    JOIN 表3
    ON 表2和表3的匹配条件;
    */

外连接:从两个表中,检索返回所有员工的姓名、部门名组成的表。

mysql> SELECT employee_id,department_name
    -> FROM employees e   # employees表为主表,departments表为从表。主表 LEFT OUTER JOIN 从表 ON 匹配条件;
    -> LEFT OUTER JOIN departments d   # 左外连接
    -> ON e.department_id = d.department_id;

Original: https://www.cnblogs.com/iot502/p/16503600.html
Author: 康康在路上
Title: 数据库_多表查询_内连接&外连接

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