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;
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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 | +------+-------+--------+------+------------+
写完这篇博客发现有点“孔乙己：茴字的四种写法的感觉”，但还是有收获的。另外，等三面通知等的好急啊!!