MySQL的JOIN（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。复制代码
CREATE TABLE t_blog(
id INT PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(50),
typeId INT
);
SELECT FROM t_blog;
+----+-------+--------+
|> +----+-------+--------+
|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)
);
SELECTFROM t_type;
+----+------------+
|> +----+------------+
|1 | C++ |
|2 | C |
|3 | Java |
|4 | C# |
|5 | Javascript |
+----+------------+
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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。
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SELECT FROM t_blog CROSS JOIN t_type;
SELECTFROM t_blog INNER JOIN t_type;
SELECT FROM t_blog,t_type;
SELECTFROM t_blog NATURE JOIN t_type;
select * from t_blog NATURA join t_type;
+----+-------+--------+----+------------+
|> +----+-------+--------+----+------------+
|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 |
+----+-------+--------+----+------------+
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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。
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SELECT FROM t_blog INNER JOIN t_type ON t_blog.typeId=t_type.id;
SELECTFROM 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
SELECTFROM t_blog JOIN t_type ON t_blog.typeId=t_type.id;
+----+-------+--------+----+------+
|>
+----+-------+--------+----+------+
|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# |
+----+-------+--------+----+------+
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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）。
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SELECT * FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id;
+----+-------+--------+------+------+
|>
+----+-------+--------+------+------+
|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 |
+----+-------+--------+------+------+
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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）。
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SELECT * FROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;
+------+-------+--------+----+------------+
|>
+------+-------+--------+----+------------+
| 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 |
+------+-------+--------+----+------------+
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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。
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SELECT FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
UNION
SELECTFROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id;
+------+-------+--------+------+------------+
|>
+------+-------+--------+------+------------+
| 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 |
+------+-------+--------+------+------------+
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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。
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SELECT * FROM t_blog INNER JOIN t_type ON t_blog.typeId =t_type.id;
+----+-------+--------+----+------+
|> +----+-------+--------+----+------+
|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。
+----+-------+--------+------------+
|>
+----+-------+--------+------------+
|1 | aaa | 1 | C++ |
|2 | bbb | 2 | C |
|3 | ccc | 3 | Java |
|4 | ddd | 4 | C# |
|5 | eee | 4 | Javascript |
+----+-------+--------+------------+
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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:SELECTFROM t_blog NATURA JOIN t_type;
笛卡尔积:SELECT * FROM t_blog NATURE JOIN t_type;
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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);
+----+-------+--------+------------+
|>
+----+-------+--------+------------+
|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);
+----+-------+--------+------------+
|>
+----+-------+--------+------------+
|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);
+----+------------+-------+--------+
|>
+----+------------+-------+--------+
|1 | C++ | aaa | 1 |
|2 | C | bbb | 2 |
|3 | Java | ccc | 3 |
|4 | C# | ddd | 4 |
|5 | Javascript | eee | 4 |
+----+------------+-------+--------+
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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。
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SELECT FROM t_blog LEFT JOIN t_type ON t_blog.typeId=t_type.id
WHERE t_type.id IS NULL;
+----+-------+--------+------+------+
|> +----+-------+--------+------+------+
|8 | hhh | NULL | NULL | NULL |
|9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
+----+-------+--------+------+------+
SELECTFROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id
WHERE t_blog.id IS NULL;
+------+-------+--------+----+------------+
|> +------+-------+--------+----+------------+
| 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
SELECTFROM t_blog RIGHT JOIN t_type ON t_blog.typeId=t_type.id
WHERE t_blog.id IS NULL;
+------+-------+--------+------+------------+
|> +------+-------+--------+------+------------+
| 8 | hhh | NULL | NULL | NULL |
| 9 | iii | NULL | NULL | NULL |
| 10 | jjj | NULL | NULL | NULL |
| NULL | NULL| NULL | 5 | Javascript |
+------+-------+--------+------+------------+
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