Mysql order by subquery id : where a.id = b.id, orders by b.id , query should not order, but it does

Question

我的目标是按距离排序结果。

我可以使用 FIELD() 来排序，但是对于 1000 个城市，它看起来并不高效，而且查询变得非常大，此外我必须使用 php 构建查询并运行多个查询。

我没有将“复杂”查询放在这里，而是对其进行了简化。

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？这实际上是我想要的，但我不明白为什么会这样。

2) 这个查询能不能写得更有效率？

到目前为止我尝试过的方法不起作用：

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 |
+----+

[编辑删除生产查询]

[编辑另外 2 个示例，我似乎是按 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，使用时相同的排序行为：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 union 选择 2 作为 id union 选择 3 作为 id) a,
（选择 3 作为 id union 选择 1 作为 id union 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。 SQL 结果集是无序的，除非您使用order by。 （或者，在 MySQL 中依赖于由group by 完成的已弃用排序。）SQL 引擎可以随意处理数据。并产生它喜欢的结果。结果顺序是任意的。

您的第一个查询不能更“高效”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：

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;

您在两个数字列表之间执行cross join - 使用（恐怖的恐怖）隐式连接语法而不是cross join。

您的order by 现在具有条件a.id = b.id desc。那么，在 MySQL 中，a.id = b.id 变成一个布尔值，它的值为 1 为真，0 为假。因此，这将订购笛卡尔积，将匹配项放在列表的顶部。它没有过滤