MySQL的JOIN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。

  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 |

     +----+------------+

笛卡尔积：CROSS JOIN

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 |

     +----+-------+--------+----+------------+

内连接：INNER JOIN

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。至于哪种好我会在MySQL的JOIN（二）：优化讲述。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#   |

    +----+-------+--------+----+------+

左连接：LEFT JOIN

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 |

    +----+-------+--------+------+------+

右连接：RIGHT JOIN

同理右连接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 |

    +------+-------+--------+----+------------+

外连接：OUTER JOIN

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 |

    +------+-------+--------+------+------------+

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 |

    +----+-------+--------+------------+

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 |

    +----+------------+-------+--------+

补充

博客开头给出的第一张图除去讲了的内连接、左连接、右连接、外连接，还有一些特殊的韦恩图，这里补充一下。

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

    WHERE t_type.id IS NULL;

    +----+-------+--------+------+------+

    | id | title | typeId | id   | name |

    +----+-------+--------+------+------+

    |  8 | hhh   |   NULL | NULL | NULL |

    |  9 | iii   |   NULL | NULL | NULL |

    | 10 | jjj   |   NULL | NULL | NULL |

    +----+-------+--------+------+------+

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

    WHERE t_blog.id IS NULL;

    +------+-------+--------+----+------------+

    | id   | title | typeId | id | name       |

    +------+-------+--------+----+------------+

    | NULL | NULL  |   NULL |  5 | Javascript |

    +------+-------+--------+----+------------+

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

    WHERE t_type.id IS NULL

    UNION

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

    WHERE t_blog.id IS NULL;

    +------+-------+--------+------+------------+

    | id   | title | typeId | id   | name       |

    +------+-------+--------+------+------------+

    |    8 | hhh   |   NULL | NULL | NULL       |

    |    9 | iii   |   NULL | NULL | NULL       |

    |   10 | jjj   |   NULL | NULL | NULL       |

    | NULL | NULL  |   NULL |    5 | Javascript |

    +------+-------+--------+------+------------+

好了，大家试一下怎么用吧，希望能帮到是的不是很熟练的童鞋。

巴特西