本文主要介绍SQL语句中ON和WHERE的区别。
1 概述
当我们使用连接操作,关联两张或多张表来返回记录时,数据库都会生成一张临时表,最后将这张临时表返回给用户。
以 LEFT JOIN 为例:在使用 LEFT JOIN 时,ON 和 WHERE 过滤条件的区别如下:
- ON 条件是在生成临时表时使用的条件,它不管 ON 中的条件是否为真,都会返回左边表中的记录;
- WHERE 条件是在临时表已经生成后,对临时表进行的过滤条件。因为此时已经没有 LEFT JOIN 的含义(必须返回左侧表的记录)了,所以如果 WHERE 条件不为真的记录就会被过滤掉。
2 示例
我们对此文中的两张表 roles 和 mount_info 进行操作,来介绍 ON 和 WHERE 之间的区别。
2.1 ON+WHERE过滤条件
ON+WHERE过滤条件语句如下:
SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id) WHERE mount_info.mount_name="sheep";
上述SQL语句的运行结果如下:
mysql> SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id) WHERE mount_info.mount_name="sheep";
+---------+------------+----------+----------+------------+---------+
| role_id | occupation | camp | mount_id | mount_name | role_id |
+---------+------------+----------+----------+------------+---------+
| 1 | warrior | alliance | 2 | sheep | 1 |
+---------+------------+----------+----------+------------+---------+
1 row in set (0.00 sec)
mysql>
分析上述SQL语句的执行过程,如下:
1. 首先,根据ON过滤条件“roles.role_id = mount_info.role_id”(相当于去掉后面的WHERE过滤条件),生成中间表,如下:
mysql> SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id);
+---------+------------+----------+----------+------------+---------+
| role_id | occupation | camp | mount_id | mount_name | role_id |
+---------+------------+----------+----------+------------+---------+
| 1 | warrior | alliance | 1 | horse | 1 |
| 1 | warrior | alliance | 2 | sheep | 1 |
| 2 | paladin | alliance | NULL | NULL | NULL |
| 3 | rogue | Horde | NULL | NULL | NULL |
+---------+------------+----------+----------+------------+---------+
4 rows in set (0.00 sec)
mysql>
2. 然后,针对上面生成的中间表,再根据WHERE过滤条件“mount_info.mount_name="sheep"”,产生最终查询结果,如下:
mysql> SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id) WHERE mount_info.mount_name="sheep";
+---------+------------+----------+----------+------------+---------+
| role_id | occupation | camp | mount_id | mount_name | role_id |
+---------+------------+----------+----------+------------+---------+
| 1 | warrior | alliance | 2 | sheep | 1 |
+---------+------------+----------+----------+------------+---------+
1 row in set (0.00 sec)
2.2 ON过滤条件
ON过滤条件语句如下:
SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id AND mount_info.mount_name = "sheep");
上述SQL语句的运行结果如下:
mysql> SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id AND mount_info.mount_name = "sheep");
+---------+------------+----------+----------+------------+---------+
| role_id | occupation | camp | mount_id | mount_name | role_id |
+---------+------------+----------+----------+------------+---------+
| 1 | warrior | alliance | 2 | sheep | 1 |
| 2 | paladin | alliance | NULL | NULL | NULL |
| 3 | rogue | Horde | NULL | NULL | NULL |
+---------+------------+----------+----------+------------+---------+
3 rows in set (0.01 sec)
mysql>
分析上述SQL语句的执行过程,如下:
根据ON过滤条件“roles.role_id = mount_info.role_id AND mount_info.mount_name = "sheep"”,生成中间表,即使ON过滤条件不为真,也会返回左表中的所有记录。
3 总结
对比下面两条ON过滤条件语句及其运行结果:
mysql> SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id);
+---------+------------+----------+----------+------------+---------+
| role_id | occupation | camp | mount_id | mount_name | role_id |
+---------+------------+----------+----------+------------+---------+
| 1 | warrior | alliance | 1 | horse | 1 |
| 1 | warrior | alliance | 2 | sheep | 1 |
| 2 | paladin | alliance | NULL | NULL | NULL |
| 3 | rogue | Horde | NULL | NULL | NULL |
+---------+------------+----------+----------+------------+---------+
4 rows in set (0.01 sec)
mysql> SELECT * FROM roles LEFT JOIN mount_info ON (roles.role_id = mount_info.role_id AND mount_info.mount_name = "sheep");
+---------+------------+----------+----------+------------+---------+
| role_id | occupation | camp | mount_id | mount_name | role_id |
+---------+------------+----------+----------+------------+---------+
| 1 | warrior | alliance | 2 | sheep | 1 |
| 2 | paladin | alliance | NULL | NULL | NULL |
| 3 | rogue | Horde | NULL | NULL | NULL |
+---------+------------+----------+----------+------------+---------+
3 rows in set (0.01 sec)
mysql>
能够发现ON过滤条件在 LEFT JOIN 模式下,相当于只增加了对于右侧表的过滤条件,而左侧表的内容,总是会全部返回的。
出现上述情况的原因在于 LEFT JOIN(以及 RIGHT JOIN、FULL JOIN )的特殊性,不管 ON 条件是否为真,数据库都会返回左侧(或右侧、左右两侧)表中的全部记录。 由于 INNER JOIN 没这样的特殊性,所以过滤条件放在 ON 中或 WHERE 中,其返回的结果是一样的。