Question

我的目标是按距离订购结果。

我可以使用FIELD（）来排序，但是使用1000的城市它看起来并不高效，而且查询变得非常大，除了我必须使用php构建查询并运行多重查询。

我在这里简化了“复杂”查询，而不是将其复杂化。

select a.id from 
(select 1 as id union select 2 as id union select 3 as id) a, 
(select 2 as id union select  3 as id union select 1 as id) b 
where  a.id  = b.id ;


Result
+----+
| id |
+----+
|  2 |
|  3 |
|  1 |
+----+

我的问题是： 1）为什么这个查询按b.id排序？这实际上就是我想要的，但我不明白为什么会这样做。

2）这个查询可以写得更有效吗？

到目前为止，我所尝试的内容无效：

select a.id from  
(select 1 as id union select 2 as id union select 3 as id) a,  
(select 2 as id union select  3 as id union select 1 as id) b  
order by  a.id  = b.id desc;

不起作用。结果：

+----+
| id |
+----+
|  1 |
|  3 |
|  1 |
|  2 |
|  2 |
|  3 |
+----+

[编辑删除了生产查询]

[编辑2个更多的例子，我似乎按b.id的选择顺序排序]

select a.id from  
(select 1 as id union select 2 as id union select 3 as id) a,  
(select  1 as id union select  2 as id union select 3 as id) b  
where  a.id  = b.id;

Result

+----+
| id |
+----+
|  1 |
|  2 |
|  3 |
+----+


select a.id from  
(select 1 as id union select 2 as id union select 3 as id) a,  
(select  3 as id union select  2 as id union select 1 as id) b  
where  a.id  = b.id;

Result

+----+
| id |
+----+
|  3 |
|  2 |
|  1 |
+----+

[编辑4，使用时的相同排序行为：按a.id IN（b.id）排序]

 select a.id from       
(select 1 as id union select 2 as id union select 3 as id) a,       
(select  1 as id      union select  2 as id union select 3 as id) b       
where  a.id  IN ( b.id );

Result

+----+
| id |
+----+
|  1 |
|  2 |
|  3 |
+----+

从中选择a.id.     （选择1作为id联合选择2作为id联合选择3作为id）a，
    （选择3作为id联合选择1作为id联合选择2作为id）b
    其中a.id IN（b.id）;

Result

+----+
| id |
+----+
|  3 |
|  1 |
|  2 |
+----+

Answer 1

第一个问题的答案很简单。除非使用order by，否则SQL结果集是无序的。（或者，在MySQL中依赖于group by所做的弃用排序。）SQL引擎可以处理数据，但它喜欢。然而产生它喜欢的结果。结果顺序是任意的。

您的第一个查询无法更有效地编写，但强烈建议使用正确的join语法：

select a.id
from  (select 1 as id union select 2 as id union select 3 as id
      ) a join
      (select 2 as id union select 3 as id union select 1 as id
      ) b  
      on a.id  = b.id;

第二个更有趣。我们来看看它：

select a.id
from  (select 1 as id union select 2 as id union select 3 as id) a,  
      (select 2 as id union select 3 as id union select 1 as id) b  
order by  a.id  = b.id desc;

你在两个数字列表之间做cross join - 使用（恐怖恐怖）隐式连接语法而不是cross join。

您的order by现在的条件为a.id = b.id desc。好吧，在MySQL中，a.id = b.id成为一个布尔值，其值为1表示true，0表示false。因此，这将订购笛卡尔积，将匹配放在列表的顶部。它没有过滤

mysql按子查询顺序排序：其中a.id = b.id，按b.id排序，查询不应该订购，但确实如此

1 个答案: