关于MySQL LEFT JOIN 你需要了解的

即使你认为自己已对 MySQL 的 LEFT JOIN 理解深刻,这篇文章肯定能让你学会点东西！

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？

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> 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)

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> 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> 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)

看看下面的示例:

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)

让我们检查一下 ON 匹配子句：
(a.id=b.id) AND (b.weight!=44) AND (b.exist=0)

我们可以把 IS NULL 子句看作是否定匹配条件。

这意味着我们将检索到以下行：
   !( 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=1

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）

看看别的示例：

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)

Matching-Conditions 与 Where-conditions 之战

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。

例如，你可以不这样写：
   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;

你可以这样写：
   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> 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)

你可以不这样写：
   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;

可以这样写：
   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> 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)

这些查询真的效果一样？

如果你只需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。

如前所属，WHERE 子句是在匹配阶段之后用来过滤的。

例如：

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)

原文链接：http://www.mysqldiary.com/mysql-left-join/

there are many accredited colleges to
how to lose weight fast Damaged Hair Care for the Black Woman

Instead of a couture dress that others can only dream of affording
christina aguilera weight lossExclusive interview with Danielle Pettee

近期文章

近期评论

文章归档

功能

链接表

关于MySQL LEFT JOIN 你需要了解的

近期文章

近期评论

标签

文章归档

功能

链接表