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 | 
