即使你认为自己已对 MySQL 的 LEFT JOIN 理解深刻,但我敢打赌,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:
01 |
mysql> CREATE
TABLE
product |
02 |
id |
03 |
amount |
04 |
PRIMARY
KEY
(id |
05 |
) ENGINE=MyISAM AUTO_INCREMENT=5 DEFAULT
CHARSET=latin1 |
06 |
|
07 |
mysql> CREATE
TABLE
product_details |
08 |
id |
09 |
weight |
10 |
exist |
11 |
PRIMARY
KEY
(id |
12 |
) ENGINE=MyISAM DEFAULT
CHARSET=latin1 |
13 |
|
14 |
mysql> INSERT
INTO
product (id,amount) |
15 |
VALUES
(1,100),(2,200),(3,300),(4,400); |
16 |
Query OK, 4 rows
affected (0.00 sec) |
17 |
Records: 4 Duplicates: 0 Warnings: 0 |
18 |
|
19 |
mysql> INSERT
INTO
product_details (id,weight,exist) |
20 |
VALUES
(2,22,0),(4,44,1),(5,55,0),(6,66,1); |
21 |
Query OK, 4 rows
affected (0.00 sec) |
22 |
Records: 4 Duplicates: 0 Warnings: 0 |
23 |
|
24 |
mysql> SELECT
* FROM
product; |
25 |
+ ----+--------+ |
26 |
| id | amount | |
27 |
+ ----+--------+ |
28 |
| 1 | 100 | |
29 |
| 2 | 200 | |
30 |
| 3 | 300 | |
31 |
| 4 | 400 | |
32 |
+ ----+--------+ |
33 |
4 rows
in
set
(0.00 sec) |
34 |
|
35 |
mysql> SELECT
* FROM
product_details; |
36 |
+ ----+--------+-------+ |
37 |
| id | weight | exist | |
38 |
+ ----+--------+-------+ |
39 |
| 2 | 22 | 0 | |
40 |
| 4 | 44 | 1 | |
41 |
| 5 | 55 | 0 | |
42 |
| 6 | 66 | 1 | |
43 |
+ ----+--------+-------+ |
44 |
4 rows
in
set
(0.00 sec) |
45 |
|
46 |
mysql> SELECT
* FROM
product LEFT
JOIN
product_details |
47 |
ON
(product.id = product_details.id); |
48 |
+ ----+--------+------+--------+-------+ |
49 |
| id | amount | id | weight | exist | |
50 |
+ ----+--------+------+--------+-------+ |
51 |
| 1 | 100 | NULL
| NULL
| NULL
| |
52 |
| 2 | 200 | 2 | 22 | 0 | |
53 |
| 3 | 300 | NULL
| NULL
| NULL
| |
54 |
| 4 | 400 | 4 | 44 | 1 | |
55 |
+ ----+--------+------+--------+-------+ |
56 |
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 |
1. SELECT
* FROM
product LEFT
JOIN
product_details |
2 |
ON
(product.id = product_details.id) |
3 |
AND
product_details.id=2; |
4 |
2. SELECT
* FROM
product LEFT
JOIN
product_details |
5 |
ON
(product.id = product_details.id) |
6 |
WHERE
product_details.id=2; |
用例子来理解最好不过了:
01 |
mysql> SELECT
* FROM
product LEFT
JOIN
product_details |
02 |
ON
(product.id = product_details.id) |
03 |
AND
product_details.id=2; |
04 |
+ ----+--------+------+--------+-------+ |
05 |
| id | amount | id | weight | exist | |
06 |
+ ----+--------+------+--------+-------+ |
07 |
| 1 | 100 | NULL
| NULL
| NULL
| |
08 |
| 2 | 200 | 2 | 22 | 0 | |
09 |
| 3 | 300 | NULL
| NULL
| NULL
| |
10 |
| 4 | 400 | NULL
| NULL
| NULL
| |
11 |
+ ----+--------+------+--------+-------+ |
12 |
4 rows
in
set
(0.00 sec) |
13 |
|
14 |
mysql> SELECT
* FROM
product LEFT
JOIN
product_details |
15 |
ON
(product.id = product_details.id) |
16 |
WHERE
product_details.id=2; |
17 |
+ ----+--------+----+--------+-------+ |
18 |
| id | amount | id | weight | exist | |
19 |
+ ----+--------+----+--------+-------+ |
20 |
| 2 | 200 | 2 | 22 | 0 | |
21 |
+ ----+--------+----+--------+-------+ |
22 |
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:
01 |
mysql> |
02 |
mysql> SELECT
* FROM
product LEFT
JOIN
product_details |
03 |
ON
product.id = product_details.id |
04 |
AND
product.amount=100; |
05 |
+ ----+--------+------+--------+-------+ |
06 |
| id | amount | id | weight | exist | |
07 |
+ ----+--------+------+--------+-------+ |
08 |
| 1 | 100 | NULL
| NULL
| NULL
| |
09 |
| 2 | 200 | NULL
| NULL
| NULL
| |
10 |
| 3 | 300 | NULL
| NULL
| NULL
| |
11 |
| 4 | 400 | NULL
| NULL
| NULL
| |
12 |
+ ----+--------+------+--------+-------+ |
13 |
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)
01 |
mysql> SELECT
* FROM
product LEFT
JOIN
product_details |
02 |
ON
(product.id = product_details.id) |
03 |
AND
product.amount=200; |
04 |
+ ----+--------+------+--------+-------+ |
05 |
| id | amount | id | weight | exist | |
06 |
+ ----+--------+------+--------+-------+ |
07 |
| 1 | 100 | NULL
| NULL
| NULL
| |
08 |
| 2 | 200 | 2 | 22 | 0 | |
09 |
| 3 | 300 | NULL
| NULL
| NULL
| |
10 |
| 4 | 400 | NULL
| NULL
| NULL
| |
11 |
+ ----+--------+------+--------+-------+ |
12 |
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。
使用 WHERE ... IS NULL 子句的 LEFT JOIN
当你使用 WHERE ... IS NULL 子句时会发生什么呢?
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。
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。
我总结了一种简单的方式来理解上述情况:
- 将 IS NULL 作为否定匹配条件
- 使用 !(A and B) == !A OR !B 逻辑判断
看看下面的示例:
01 |
mysql> SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
02 |
ON
a.id=b.id AND
b.weight!=44 AND
b.exist=0 |
03 |
WHERE
b.id IS
NULL ; |
04 |
+ ----+--------+ |
05 |
| id | amount | |
06 |
+ ----+--------+ |
07 |
| 1 | 100 | |
08 |
| 3 | 300 | |
09 |
| 4 | 400 | |
10 |
+ ----+--------+ |
11 |
3 rows
in
set
(0.00 sec) |
让我们检查一下 ON 匹配子句:
1 |
(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:
1 |
!( exist(b.id that equals to
a.id) AND
b.weight !=44 AND
b.exist=0 ) |
2 |
!exist(b.id that equals to
a.id) || !(b.weight !=44) || !(b.exist=0) |
3 |
!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:
01 |
mysql> SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
02 |
ON
a.id=b.id AND
b.weight!=44 AND
b.exist=1 |
03 |
WHERE
b.id IS
NULL ; |
04 |
+ ----+--------+ |
05 |
| id | amount | |
06 |
+ ----+--------+ |
07 |
| 1 | 100 | |
08 |
| 2 | 200 | |
09 |
| 3 | 300 | |
10 |
| 4 | 400 | |
11 |
+ ----+--------+ |
12 |
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:
1 |
SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
2 |
ON
a.id=b.id AND
b.weight!=44 AND
b.exist=0 |
3 |
WHERE
b.id IS
NULL ; |
你可以这样写:
1 |
SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
2 |
ON
a.id=b.id |
3 |
WHERE
b.id is
null
OR
b.weight=44 OR
b.exist=1; |
01 |
mysql> SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
02 |
ON
a.id=b.id |
03 |
WHERE
b.id is
null
OR
b.weight=44 OR
b.exist=1; |
04 |
+ ----+--------+ |
05 |
| id | amount | |
06 |
+ ----+--------+ |
07 |
| 1 | 100 | |
08 |
| 3 | 300 | |
09 |
| 4 | 400 | |
10 |
+ ----+--------+ |
11 |
3 rows
in
set
(0.00 sec) |
你可以不这样写:
1 |
SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
2 |
ON
a.id=b.id AND
b.weight!=44 AND
b.exist!=0 |
3 |
WHERE
b.id IS
NULL ; |
可以这样写:
1 |
SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
2 |
ON
a.id=b.id |
3 |
WHERE
b.id is
null
OR
b.weight=44 OR
b.exist=0; |
01 |
mysql> SELECT
a.* FROM
product a LEFT
JOIN
product_details b |
02 |
ON
a.id=b.id |
03 |
WHERE
b.id is
null
OR
b.weight=44 OR
b.exist=0; |
04 |
+ ----+--------+ |
05 |
| id | amount | |
06 |
+ ----+--------+ |
07 |
| 1 | 100 | |
08 |
| 2 | 200 | |
09 |
| 3 | 300 | |
10 |
| 4 | 400 | |
11 |
+ ----+--------+ |
12 |
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:
01 |
mysql> SELECT
* FROM
product a LEFT
JOIN
product_details b |
02 |
ON
a.id=b.id AND
b.weight!=44 AND
b.exist=1 |
03 |
WHERE
b.id is
null ; |
04 |
+ ----+--------+------+--------+-------+ |
05 |
| id | amount | id | weight | exist | |
06 |
+ ----+--------+------+--------+-------+ |
07 |
| 1 | 100 | NULL
| NULL
| NULL
| |
08 |
| 2 | 200 | NULL
| NULL
| NULL
| |
09 |
| 3 | 300 | NULL
| NULL
| NULL
| |
10 |
| 4 | 400 | NULL
| NULL
| NULL
| |
11 |
+ ----+--------+------+--------+-------+ |
12 |
4 rows
in
set
(0.00 sec) |
13 |
|
14 |
mysql> SELECT
* FROM
product a LEFT
JOIN
product_details b |
15 |
ON
a.id=b.id |
16 |
WHERE
b.id IS
NULL
OR
b.weight=44 OR
b.exist=0; |
17 |
+ ----+--------+------+--------+-------+ |
18 |
| id | amount | id | weight | exist | |
19 |
+ ----+--------+------+--------+-------+ |
20 |
| 1 | 100 | NULL
| NULL
| NULL
| |
21 |
| 2 | 200 | 2 | 22 | 0 | |
22 |
| 3 | 300 | NULL
| NULL
| NULL
| |
23 |
| 4 | 400 | 4 | 44 | 1 | |
24 |
+ ----+--------+------+--------+-------+ |
25 |
4 rows
in
set
(0.00 sec) |
总附注:
如果你使用 LEFT JOIN 来寻找在一些表中不存在的记录,你需要做下面的测试: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)。