MySql中如何使用JOIN

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。

先创建两个表，下面用于示例

    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。笛卡尔积是将A表中的每个记录与B表中的每个记录进行组合。因此，当A表中有n条记录且B表中有m条记录时，笛卡尔积操作得到的结果将会有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。从数学的角度来说，这就是要计算两个表的相交部分；而从笛卡尔积的角度来看，就是要从笛卡尔积中筛选出满足ON子句条件的记录。有INNER JOIN，WHERE（等值连接），STRAIGHT_JOIN,JOIN(省略INNER)四种写法。

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。MySQL不支持OUTER JOIN，但是我们可以通过将左连接和右连接的结果进行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);    |  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 |

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