《Mysql必读深入理解mysql之left join 使用详解》要点：
本文介绍了Mysql必读深入理解mysql之left join 使用详解，希望对您有用。如果有疑问，可以联系我们。
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办法
Matching-Conditions 与 Where-conditions 的不同MYSQL必读
关于 “A LEFT JOIN B ON 条件表达式” 的一点提醒MYSQL必读
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.MYSQL必读
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MYSQL必读
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.MYSQL必读
让我们看一个 LFET JOIN 示例：MYSQL必读
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)MYSQL必读
ON 子句和 WHERE 子句有什么不同?MYSQL必读
一个问题：下面两个查询的结果集有什么不同么?MYSQL必读
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;MYSQL必读
用例子来理解最好不过了：MYSQL必读
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)MYSQL必读
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.MYSQL必读
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.MYSQL必读
再来看一些示例：MYSQL必读
mysql>
mysql> SELECT * FROM product LEFT JOIN product_details
ON product.id = product_details.id
AND product.amount=100;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | NULL | NULL | NULL |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
4 rows in set (0.00 sec)MYSQL必读
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)MYSQL必读
mysql> SELECT * FROM product LEFT JOIN product_details
ON (product.id = product_details.id)
AND product.amount=200;
+----+--------+------+--------+-------+
| 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 (0.01 sec)MYSQL必读
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.MYSQL必读
使用 WHERE ... IS NULL 子句的 LEFT JOINMYSQL必读
当你使用 WHERE ... IS NULL 子句时会发生什么呢?MYSQL必读
如前所述,WHERE 条件查询发生在 匹配阶段之后,这意味着 WHERE ... IS NULL 子句将从匹配阶段后的数据中过滤掉不满足匹配条件的数据行.MYSQL必读
纸面上看起来很清楚,但是当你在 ON 子句中使用多个条件时就会感到困惑了.MYSQL必读
我总结了一种简单的方式来理解上述情况：MYSQL必读
将 IS NULL 作为否定匹配条件
使用 !(A and B) == !A OR !B 逻辑判断MYSQL必读
看看下面的示例：MYSQL必读
mysql> SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id AND b.weight!=44 AND b.exist=0
WHERE b.id IS NULL;
+----+--------+
| id | amount |
+----+--------+
| 1 | 100 |
| 3 | 300 |
| 4 | 400 |
+----+--------+
3 rows in set (0.00 sec)MYSQL必读
让我们检查一下 ON 匹配子句：MYSQL必读
(a.id=b.id) AND (b.weight!=44) AND (b.exist=0)MYSQL必读
我们可以把 IS NULL 子句 看作是否定匹配条件.MYSQL必读
这意味着我们将检索到以下行：MYSQL必读
!( exist(b.id that equals to a.id) AND b.weight !=44 AND b.exist=0 )
!exist(b.id that equals to a.id) || !(b.weight !=44) || !(b.exist=0)
!exist(b.id that equals to a.id) || b.weight =44 || b.exist=1MYSQL必读
就像在C语言中的逻辑 AND 和 逻辑 OR表达式一样,其操作数是从左到右求值的.如果第一个参数做够判断操作结果,那么第二个参数便不会被计算求值(短路效果)MYSQL必读
看看别的示例：MYSQL必读
mysql> SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id AND b.weight!=44 AND b.exist=1
WHERE b.id IS NULL;
+----+--------+
| id | amount |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
| 4 | 400 |
+----+--------+
4 rows in set (0.00 sec)MYSQL必读
Matching-Conditions 与 Where-conditions 之战MYSQL必读
如果你吧基本的查询条件放在 ON 子句中,把剩下的否定条件放在 WHERE 子句中,那么你会获得相同的结果.MYSQL必读
例如,你可以不这样写：MYSQL必读
SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id AND b.weight!=44 AND b.exist=0
WHERE b.id IS NULL;MYSQL必读
你可以这样写：MYSQL必读
SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id
WHERE b.id is null OR b.weight=44 OR b.exist=1;MYSQL必读
mysql> SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id
WHERE b.id is null OR b.weight=44 OR b.exist=1;
+----+--------+
| id | amount |
+----+--------+
| 1 | 100 |
| 3 | 300 |
| 4 | 400 |
+----+--------+
3 rows in set (0.00 sec)MYSQL必读
你可以不这样写：MYSQL必读
SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id AND b.weight!=44 AND b.exist!=0
WHERE b.id IS NULL;MYSQL必读
可以这样写：MYSQL必读
SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id
WHERE b.id is null OR b.weight=44 OR b.exist=0;MYSQL必读
mysql> SELECT a.* FROM product a LEFT JOIN product_details b
ON a.id=b.id
WHERE b.id is null OR b.weight=44 OR b.exist=0;
+----+--------+
| id | amount |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
| 4 | 400 |
+----+--------+
4 rows in set (0.00 sec)MYSQL必读
这些查询真的效果一样?MYSQL必读
如果你只必要第一个表中的数据的话,这些查询会返回相同的结果集.有一种情况就是,如果你从 LEFT JOIN的表中检索数据时,查询的结果就不同了.MYSQL必读
如前所属,WHERE 子句是在匹配阶段之后用来过滤的.MYSQL必读
例如：MYSQL必读
mysql> SELECT * FROM product a LEFT JOIN product_details b
ON a.id=b.id AND b.weight!=44 AND b.exist=1
WHERE b.id is null;
+----+--------+------+--------+-------+
| id | amount | id | weight | exist |
+----+--------+------+--------+-------+
| 1 | 100 | NULL | NULL | NULL |
| 2 | 200 | NULL | NULL | NULL |
| 3 | 300 | NULL | NULL | NULL |
| 4 | 400 | NULL | NULL | NULL |
+----+--------+------+--------+-------+
4 rows in set (0.00 sec)
mysql> SELECT * FROM product a LEFT JOIN product_details b
ON a.id=b.id
WHERE b.id IS NULL OR b.weight=44 OR b.exist=0;
+----+--------+------+--------+-------+
| 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)MYSQL必读
总附注：MYSQL必读
如果你使用 LEFT JOIN 来寻找在一些表中不存在的记录,你必要做下面的测试：WHERE 部分的 col_name IS NULL(其中 col_name 列被定义为 NOT NULL),MYSQL 在查询到一条匹配 LEFT JOIN 条件后将停止搜索更多行(在一个特定的组合键下). MYSQL必读
《Mysql必读深入理解mysql之left join 使用详解》是否对您有启发，欢迎查看更多与《Mysql必读深入理解mysql之left join 使用详解》相关教程，学精学透。维易PHP学院为您提供精彩教程。
