深入理解mysql之left join 使用详解

ON 子句与 WHERE 子句的不同
一种更好地理解带有 WHERE ... IS NULL 子句的复杂匹配条件的简单方法
Matching-Conditions 与 Where-conditions 的不同

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。

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：

mysql> CREATE TABLE `product` (
`id` int(10) unsigned NOT NULL auto_increment,
`amount` int(10) unsigned default NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT CHARSET=latin1

mysql> CREATE TABLE `product_details` (
`id` int(10) unsigned NOT NULL,
`weight` int(10) unsigned default NULL,
`exist` int(10) unsigned default NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

mysql> INSERT INTO product (id,amount)
       VALUES (1,100),(2,200),(3,300),(4,400);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0

mysql> INSERT INTO product_details (id,weight,exist)
       VALUES (2,22,0),(4,44,1),(5,55,0),(6,66,1);
Query OK, 4 rows affected (0.00 sec)
Records: 4 Duplicates: 0 Warnings: 0

mysql> SELECT * FROM product;
+----+--------+
| id | amount |
+----+--------+
| 1 |    100 |
| 2 |    200 |
| 3 |    300 |
| 4 |    400 |
+----+--------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM product_details;
+----+--------+-------+
| id | weight | exist |
+----+--------+-------+
| 2 |     22 |     0 |
| 4 |     44 |     1 |
| 5 |     55 |     0 |
| 6 |     66 |     1 |
+----+--------+-------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM product LEFT JOIN product_details
       ON (product.id = product_details.id);
+----+--------+------+--------+-------+
| id | amount | id   | weight | exist |
+----+--------+------+--------+-------+
| 1 |    100 | NULL |   NULL | NULL |
| 2 |    200 |    2 |     22 |     0 |
| 3 |    300 | NULL |   NULL | NULL |
| 4 |    400 |    4 |     44 |     1 |
+----+--------+------+--------+-------+
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？

1. SELECT * FROM product LEFT JOIN product_details
         ON (product.id = product_details.id)
         AND   product_details.id=2;
2. SELECT * FROM product LEFT JOIN product_details
         ON (product.id = product_details.id)
         WHERE product_details.id=2;

用例子来理解最好不过了：

mysql> SELECT * FROM product LEFT JOIN product_details
       ON (product.id = product_details.id)
       AND product_details.id=2;
+----+--------+------+--------+-------+
| id | amount | id   | weight | exist |
+----+--------+------+--------+-------+
| 1 |    100 | NULL |   NULL | NULL |
| 2 |    200 |    2 |     22 |     0 |
| 3 |    300 | NULL |   NULL | NULL |
| 4 |    400 | NULL |   NULL | NULL |
+----+--------+------+--------+-------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM product LEFT JOIN product_details
       ON (product.id = product_details.id)
       WHERE product_details.id=2;
+----+--------+----+--------+-------+
| id | amount | id | weight | exist |
+----+--------+----+--------+-------+
| 2 |    200 | 2 |     22 |     0 |
+----+--------+----+--------+-------+
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：

mysql>
mysql> SELECT * FROM product LEFT JOIN product]4(~rjV#zs4(zsrjV6jwg~Sn{n3jzsnr'7bzs81P)=%;jV6^jzsB34(&7&IC>br27bB;Rvj4(4(M1 PI=4Ё1P)=%8х899!IЁЁ10U10U1010U10U1010U10U1010U10U10ЁM1 PI=4Ё1P)=%8х8!I%L9U10=H=HЁЁ10U10U1010U10U10ЁЁ4(Σ4(zsP)=%8vcrj3kvjWI"j}%L9U10}"_k9=P9U10ME0r~"v241P)=%8vB;sBsnk3&疺kjB#R