【Mysql】Mysql的Join完全解读

2018-11-14 本文已影响7人慕凌峰

一、用法

JOIN的含义就如英文单词“join”一样，连接两张表，大致分为内连接，外连接，右连接，左连接，自然连接。这里描述先甩出一张用烂了的图，然后插入测试数据。

join

用例表

CREATE TABLE t_blog(
        id INT PRIMARY KEY AUTO_INCREMENT,
        title VARCHAR(50),
        typeId INT
);

SELECT * FROM t_blog;
+----+-------+--------+
| id | title | typeId |
+----+-------+--------+
|  1 | aaa   |      1 |
|  2 | bbb   |      2 |
|  3 | ccc   |      3 |
|  4 | ddd   |      4 |
|  5 | eee   |      4 |
|  6 | fff   |      3 |
|  7 | ggg   |      2 |
|  8 | hhh   |   NULL |
|  9 | iii   |   NULL |
| 10 | jjj   |   NULL |
+----+-------+--------+

-- 博客的类别
CREATE TABLE t_type(
    id INT PRIMARY KEY AUTO_INCREMENT,
    name VARCHAR(20)
);

SELECT * FROM t_type;
+----+------------+
| id | name       |
+----+------------+
|  1 | C++        |
|  2 | C          |
|  3 | Java       |
|  4 | C#         |
|  5 | Javascript |
+----+------------+

1、笛卡尔积：CROSS JOIN

A 表：n条记录，B 表：m条记录===> n * m 条记录

要理解各种JOIN首先要理解笛卡尔积。笛卡尔积就是将A表的每一条记录与B表的每一条记录强行拼在一起。所以，如果A表有n条记录，B表有m条记录，笛卡尔积产生的结果就会产生 n*m 条记录。下面的例子，t_blog有10条记录，t_type有5条记录，所有他们俩的笛卡尔积有50条记录。有五种产生笛卡尔积的方式如下。

SELECT * FROM t_blog CROSS JOIN t_type;
SELECT * FROM t_blog INNER JOIN t_type;
SELECT * FROM t_blog,t_type;
SELECT * FROM t_blog NATURE JOIN t_type;
select * from t_blog NATURA join t_type;
+----+-------+--------+----+------------+
| id | title | typeId | id | name       |
+----+-------+--------+----+------------+
|  1 | aaa   |      1 |  1 | C++        |
|  1 | aaa   |      1 |  2 | C          |
|  1 | aaa   |      1 |  3 | Java       |
|  1 | aaa   |      1 |  4 | C#         |
|  1 | aaa   |      1 |  5 | Javascript |
|  2 | bbb   |      2 |  1 | C++        |
|  2 | bbb   |      2 |  2 | C          |
|  2 | bbb   |      2 |  3 | Java       |
|  2 | bbb   |      2 |  4 | C#         |
|  2 | bbb   |      2 |  5 | Javascript |
|  3 | ccc   |      3 |  1 | C++        |
|  3 | ccc   |      3 |  2 | C          |
|  3 | ccc   |      3 |  3 | Java       |
|  3 | ccc   |      3 |  4 | C#         |
|  3 | ccc   |      3 |  5 | Javascript |
|  4 | ddd   |      4 |  1 | C++        |
|  4 | ddd   |      4 |  2 | C          |
|  4 | ddd   |      4 |  3 | Java       |
|  4 | ddd   |      4 |  4 | C#         |
|  4 | ddd   |      4 |  5 | Javascript |
|  5 | eee   |      4 |  1 | C++        |
|  5 | eee   |      4 |  2 | C          |
|  5 | eee   |      4 |  3 | Java       |
|  5 | eee   |      4 |  4 | C#         |
|  5 | eee   |      4 |  5 | Javascript |
|  6 | fff   |      3 |  1 | C++        |
|  6 | fff   |      3 |  2 | C          |
|  6 | fff   |      3 |  3 | Java       |
|  6 | fff   |      3 |  4 | C#         |
|  6 | fff   |      3 |  5 | Javascript |
|  7 | ggg   |      2 |  1 | C++        |
|  7 | ggg   |      2 |  2 | C          |
|  7 | ggg   |      2 |  3 | Java       |
|  7 | ggg   |      2 |  4 | C#         |
|  7 | ggg   |      2 |  5 | Javascript |
|  8 | hhh   |   NULL |  1 | C++        |
|  8 | hhh   |   NULL |  2 | C          |
|  8 | hhh   |   NULL |  3 | Java       |
|  8 | hhh   |   NULL |  4 | C#         |
|  8 | hhh   |   NULL |  5 | Javascript |
|  9 | iii   |   NULL |  1 | C++        |
|  9 | iii   |   NULL |  2 | C          |
|  9 | iii   |   NULL |  3 | Java       |
|  9 | iii   |   NULL |  4 | C#         |
|  9 | iii   |   NULL |  5 | Javascript |
| 10 | jjj   |   NULL |  1 | C++        |
| 10 | jjj   |   NULL |  2 | C          |
| 10 | jjj   |   NULL |  3 | Java       |
| 10 | jjj   |   NULL |  4 | C#         |
| 10 | jjj   |   NULL |  5 | Javascript |
+----+-------+--------+----+------------+

2、内连接：INNER JOIN

获得两个表的交集

内连接INNER JOIN是最常用的连接操作。从数学的角度讲就是求两个表的交集，从笛卡尔积的角度讲就是从笛卡尔积中挑出ON子句条件成立的记录。有INNER JOIN，WHERE（等值连接），STRAIGHT_JOIN,JOIN(省略INNER)四种写法。示例如下。

SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId=t_type.id;
SELECT * FROM t_blog,t_type WHERE t_blog.typeId=t_type.id;
SELECT * FROM t_blog STRAIGHT_JOIN t_type ON t_blog.typeId=t_type.id; --注意STRIGHT_JOIN有个下划线
SELECT * FROM t_blog JOIN t_type ON t_blog.typeId=t_type.id;

+----+-------+--------+----+------+
| id | title | typeId | id | name |
+----+-------+--------+----+------+
|  1 | aaa   |      1 |  1 | C++  |
|  2 | bbb   |      2 |  2 | C    |
|  7 | ggg   |      2 |  2 | C    |
|  3 | ccc   |      3 |  3 | Java |
|  6 | fff   |      3 |  3 | Java |
|  4 | ddd   |      4 |  4 | C#   |
|  5 | eee   |      4 |  4 | C#   |
+----+-------+--------+----+------+

3、左连接：LEFT JOIN

两个表的交集 + 左表剩下的数据

左连接LEFT JOIN的含义就是求两个表的交集外加左表剩下的数据。依旧从笛卡尔积的角度讲，就是先从笛卡尔积中挑出ON子句条件成立的记录，然后加上左表中剩余的记录（见最后三条）。

SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id;

+----+-------+--------+------+------+
| id | title | typeId | id   | name |
+----+-------+--------+------+------+
|  1 | aaa   |      1 |    1 | C++  |
|  2 | bbb   |      2 |    2 | C    |
|  7 | ggg   |      2 |    2 | C    |
|  3 | ccc   |      3 |    3 | Java |
|  6 | fff   |      3 |    3 | Java |
|  4 | ddd   |      4 |    4 | C#   |
|  5 | eee   |      4 |    4 | C#   |
|  8 | hhh   |   NULL | NULL | NULL |
|  9 | iii   |   NULL | NULL | NULL |
| 10 | jjj   |   NULL | NULL | NULL |
+----+-------+--------+------+------+

4、右连接：RIGHT JOIN

两个表的交集 + 右表剩下的数据

右连接RIGHT JOIN就是求两个表的交集外加右表剩下的数据。再次从笛卡尔积的角度描述，右连接就是从笛卡尔积中挑出ON子句条件成立的记录，然后加上右表中剩余的记录（见最后一条）。

SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;

+------+-------+--------+----+------------+
| id   | title | typeId | id | name       |
+------+-------+--------+----+------------+
|    1 | aaa   |      1 |  1 | C++        |
|    2 | bbb   |      2 |  2 | C          |
|    3 | ccc   |      3 |  3 | Java       |
|    4 | ddd   |      4 |  4 | C#         |
|    5 | eee   |      4 |  4 | C#         |
|    6 | fff   |      3 |  3 | Java       |
|    7 | ggg   |      2 |  2 | C          |
| NULL | NULL  |   NULL |  5 | Javascript |
+------+-------+--------+----+------------+

5、外连接：OUTER JOIN

求两个表的并集

外连接就是求两个集合的并集。从笛卡尔积的角度讲就是从笛卡尔积中挑出ON子句条件成立的记录，然后加上左表中剩余的记录，最后加上右表中剩余的记录。另外MySQL不支持OUTER JOIN，但是我们可以对左连接和右连接的结果做UNION操作来实现。

SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
    UNION
    SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;

    +------+-------+--------+------+------------+
    | id   | title | typeId | id   | name       |
    +------+-------+--------+------+------------+
    |    1 | aaa   |      1 |    1 | C++        |
    |    2 | bbb   |      2 |    2 | C          |
    |    7 | ggg   |      2 |    2 | C          |
    |    3 | ccc   |      3 |    3 | Java       |
    |    6 | fff   |      3 |    3 | Java       |
    |    4 | ddd   |      4 |    4 | C#         |
    |    5 | eee   |      4 |    4 | C#         |
    |    8 | hhh   |   NULL | NULL | NULL       |
    |    9 | iii   |   NULL | NULL | NULL       |
    |   10 | jjj   |   NULL | NULL | NULL       |
    | NULL | NULL  |   NULL |    5 | Javascript |
    +------+-------+--------+------+------------+

6、自然连接：NATURE JOIN

自然连接就是USING子句的简化版，它找出两个表中相同的列作为连接条件进行连接。有左自然连接，右自然连接和普通自然连接之分。在t_blog和t_type示例中，两个表相同的列是id，所以会拿id作为连接条件。
另外千万分清下面三条语句的区别
- 自然连接:SELECT * FROM t_blog NATURAL JOIN t_type;
- 笛卡尔积:SELECT * FROM t_blog NATURA JOIN t_type;
- 笛卡尔积:SELECT * FROM t_blog NATURE JOIN t_type;

SELECT * FROM t_blog NATURAL JOIN t_type;
    SELECT t_blog.id,title,typeId,t_type.name FROM t_blog,t_type WHERE t_blog.id=t_type.id;
    SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type ON t_blog.id=t_type.id;
    SELECT t_blog.id,title,typeId,t_type.name FROM t_blog INNER JOIN t_type USING(id);

    +----+-------+--------+------------+
    | id | title | typeId | name       |
    +----+-------+--------+------------+
    |  1 | aaa   |      1 | C++        |
    |  2 | bbb   |      2 | C          |
    |  3 | ccc   |      3 | Java       |
    |  4 | ddd   |      4 | C#         |
    |  5 | eee   |      4 | Javascript |
    +----+-------+--------+------------+

    SELECT * FROM t_blog NATURAL LEFT JOIN t_type;
    SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type ON t_blog.id=t_type.id;
    SELECT t_blog.id,title,typeId,t_type.name FROM t_blog LEFT JOIN t_type USING(id);

    +----+-------+--------+------------+
    | id | title | typeId | name       |
    +----+-------+--------+------------+
    |  1 | aaa   |      1 | C++        |
    |  2 | bbb   |      2 | C          |
    |  3 | ccc   |      3 | Java       |
    |  4 | ddd   |      4 | C#         |
    |  5 | eee   |      4 | Javascript |
    |  6 | fff   |      3 | NULL       |
    |  7 | ggg   |      2 | NULL       |
    |  8 | hhh   |   NULL | NULL       |
    |  9 | iii   |   NULL | NULL       |
    | 10 | jjj   |   NULL | NULL       |
    +----+-------+--------+------------+

    SELECT * FROM t_blog NATURAL RIGHT JOIN t_type;
    SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type ON t_blog.id=t_type.id;
    SELECT t_blog.id,title,typeId,t_type.name FROM t_blog RIGHT JOIN t_type USING(id);

    +----+------------+-------+--------+
    | id | name       | title | typeId |
    +----+------------+-------+--------+
    |  1 | C++        | aaa   |      1 |
    |  2 | C          | bbb   |      2 |
    |  3 | Java       | ccc   |      3 |
    |  4 | C#         | ddd   |      4 |
    |  5 | Javascript | eee   |      4 |
    +----+------------+-------+--------+

USING子句

MySQL中连接SQL语句中，ON子句的语法格式为：table1.column_name = table2.column_name。当模式设计对联接表的列采用了相同的命名样式时，就可以使用 USING 语法来简化 ON 语法，格式为：USING(column_name)。
所以，USING的功能相当于ON，区别在于USING指定一个属性名用于连接两个表，而ON指定一个条件。另外，SELECT * 时，USING会去除USING指定的列，而ON不会。实例如下

SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId =t_type.id;
    +----+-------+--------+----+------+
    | id | title | typeId | id | name |
    +----+-------+--------+----+------+
    |  1 | aaa   |      1 |  1 | C++  |
    |  2 | bbb   |      2 |  2 | C    |
    |  7 | ggg   |      2 |  2 | C    |
    |  3 | ccc   |      3 |  3 | Java |
    |  6 | fff   |      3 |  3 | Java |
    |  4 | ddd   |      4 |  4 | C#   |
    |  5 | eee   |      4 |  4 | C#   |
    +----+-------+--------+----+------+


SELECT * FROM t_blog INNER JOIN t_type USING(typeId);
ERROR 1054 (42S22): Unknown column 'typeId' in 'from clause'
SELECT * FROM t_blog INNER JOIN t_type USING(id); 

--应为t_blog的typeId与t_type的id不同名，无法用Using，这里用id代替下。

    +----+-------+--------+------------+
    | id | title | typeId | name       |
    +----+-------+--------+------------+
    |  1 | aaa   |      1 | C++        |
    |  2 | bbb   |      2 | C          |
    |  3 | ccc   |      3 | Java       |
    |  4 | ddd   |      4 | C#         |
    |  5 | eee   |      4 | Javascript |
    +----+-------+--------+------------+

【Mysql】Mysql的Join完全解读

一、用法

用例表

1、笛卡尔积：CROSS JOIN

2、内连接：INNER JOIN

3、左连接：LEFT JOIN

4、右连接：RIGHT JOIN

5、外连接：OUTER JOIN

6、自然连接：NATURE JOIN

USING子句

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