MySQL的连接查询：left join , right join , join

转载：https://blog.csdn.net/lp_cq242/article/details/79942457

 

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。

 

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。

 

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.idUNIONSELECT * 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 |+----+------------+-------+--------+

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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);+----+-------+--------+------------+| 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 |+--