关于 “A LEFT JOIN B ON 条件表达式” 的一点提醒
ON 条件（“A LEFT JOIN B ON 条件表达式”中的ON）用来决定如何从 B 表中检索数据行，即使on中包含有A表中的列的限制条件，也不会过滤A的任何数据（A的数据只会通过where过滤）。
如果 B 表中没有任何一行数据匹配 ON 的条件,将会额外生成一行所有列为 NULL 的数据
在匹配阶段 WHERE 子句的条件都不会被使用。仅在匹配阶段完成以后，WHERE 子句条件才会被使用。它将从匹配阶段产生的数据中检索过滤。
让我们看一个 LFET JOIN 示例：
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mysql> CREATE TABLE `product` ( |
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`id` int(10) unsigned NOT NULL auto_increment, |
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`amount` int(10) unsigned default NULL, |
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) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=latin1 |
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mysql> CREATE TABLE `product_details` ( |
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`id` int(10) unsigned NOT NULL, |
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`weight` int(10) unsigned default NULL, |
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`exist` int(10) unsigned default NULL, |
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) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
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mysql> INSERT INTO product (id,amount) |
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VALUES (1,100),(2,200),(3,300),(4,400); |
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Query OK, 4 rows affected (0.00 sec) |
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Records: 4 Duplicates: 0 Warnings: 0 |
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mysql> INSERT INTO product_details (id,weight,exist) |
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VALUES (2,22,0),(4,44,1),(5,55,0),(6,66,1); |
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Query OK, 4 rows affected (0.00 sec) |
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Records: 4 Duplicates: 0 Warnings: 0 |
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mysql> SELECT * FROM product; |
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4 rows in set (0.00 sec) |
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mysql> SELECT * FROM product_details; |
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+----+--------+-------+ |
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| id | weight | exist | |
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+----+--------+-------+ |
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+----+--------+-------+ |
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4 rows in set (0.00 sec) |
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mysql> SELECT * FROM product LEFT JOIN product_details |
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ON (product.id = product_details.id); |
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+----+--------+------+--------+-------+ |
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| id | amount | id | weight | exist | |
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+----+--------+------+--------+-------+ |
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| 1 | 100 | NULL | NULL | NULL | |
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| 2 | 200 | 2 | 22 | 0 | |
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| 3 | 300 | NULL | NULL | NULL | |
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| 4 | 400 | 4 | 44 | 1 | |
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+----+--------+------+--------+-------+ |
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4 rows in set (0.00 sec) |
ON 子句和 WHERE 子句有什么不同？
一个问题：下面两个查询的结果集有什么不同么？
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1. SELECT * FROM product LEFT JOIN product_details |
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ON (product.id = product_details.id) |
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AND product_details.id=2; |
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2. SELECT * FROM product LEFT JOIN product_details |
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ON (product.id = product_details.id) |
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WHERE product_details.id=2; |
用例子来理解最好不过了：
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mysql> SELECT * FROM product LEFT JOIN product_details |
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ON (product.id = product_details.id) |
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AND product_details.id=2; |
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+----+--------+------+--------+-------+ |
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| id | amount | id | weight | exist | |
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+----+--------+------+--------+-------+ |
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| 1 | 100 | NULL | NULL | NULL | |
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| 2 | 200 | 2 | 22 | 0 | |
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| 3 | 300 | NULL | NULL | NULL | |
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| 4 | 400 | NULL | NULL | NULL | |
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+----+--------+------+--------+-------+ |
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4 rows in set (0.00 sec) |
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mysql> SELECT * FROM product LEFT JOIN product_details |
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ON (product.id = product_details.id) |
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WHERE product_details.id=2; |
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+----+--------+----+--------+-------+ |
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| id | amount | id | weight | exist | |
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+----+--------+----+--------+-------+ |
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| 2 | 200 | 2 | 22 | 0 | |
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+----+--------+----+--------+-------+ |
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1 row in set (0.01 sec) |
第一条查询使用 ON 条件决定了从 LEFT JOIN的 product_details表中检索符合的所有数据行。
第二条查询做了简单的LEFT JOIN，然后使用 WHERE 子句从 LEFT JOIN的数据中过滤掉不符合条件的数据行。
再来看一些示例：
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mysql> SELECT * FROM product LEFT JOIN product_details |
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ON product.id = product_details.id |
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AND product.amount=100; |
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+----+--------+------+--------+-------+ |
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| id | amount | id | weight | exist | |
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+----+--------+------+--------+-------+ |
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| 1 | 100 | NULL | NULL | NULL | |
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| 2 | 200 | NULL | NULL | NULL | |
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| 3 | 300 | NULL | NULL | NULL | |
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| 4 | 400 | NULL | NULL | NULL | |
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+----+--------+------+--------+-------+ |
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4 rows in set (0.00 sec) |
所有来自product表的数据行都被检索到了，但没有在product_details表中匹配到记录（product.id = product_details.id AND product.amount=100 条件并没有匹配到任何数据）
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mysql> SELECT * FROM product LEFT JOIN product_details |
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ON (product.id = product_details.id) |
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AND product.amount=200; |
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+----+--------+------+--------+-------+ |
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| id | amount | id | weight | exist | |
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+----+--------+------+--------+-------+ |
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| 1 | 100 | NULL | NULL | NULL | |
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| 2 | 200 | 2 | 22 | 0 | |
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| 3 | 300 | NULL | NULL | NULL | |
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| 4 | 400 | NULL | NULL | NULL | |
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+----+--------+------+--------+-------+ |
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4 rows in set (0.01 sec) |
同样，所有来自product表的数据行都被检索到了，有一条数据匹配到了。
原文地址：http://blog.csdn.net/longyulu/article/details/7924833
MySQL的left join中on与where的区别
时间： 2025-01-19 17:36:48
