无法理解mysql查询中使用聚合函数的结果

我在数据库中有一个名为cities的表，其中有id、name、country_id、lat（纬度）和lng（经度）。 在我的应用程序中，我得到了用户的位置，然后尝试找出哪个城市离用户的位置最近，所以首先我做了如下操作：

select * , ( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities

+----+------------------+------------+---------------+---------------+----------------------+
| id | name             | country_id | lat           | lng           | min_dis              |
+----+------------------+------------+---------------+---------------+----------------------+
|  1 | Cairo            |         61 | 30.0444196000 | 31.2357116000 |   1.7642055948326205 |
|  2 | Alexandria       |         61 | 31.2000924000 | 29.9187387000 | 0.012723270627083274 |
|  3 | Tanta            |         61 | 30.7865086000 | 31.0003757000 |   1.1707286078440424 |
|  7 | North Coast      |         61 | 28.0488161000 | 34.4371483000 |    5.521208240105792 |
|  8 | Marsa Matruh     |         61 | 31.3543445000 | 27.2373159000 |    2.673858069059212 |
|  9 | Hurghada         |         61 | 27.2578957000 | 33.8116067000 |    5.552079415567052 |
| 10 | Ismailia         |         61 | 30.5964923000 | 32.2714587000 |   2.4415049795085366 |
| 11 | Ain ElSokhna     |         61 | 29.5927778000 | 32.3416667000 |    2.920079170546876 |
| 12 | El Mansoura      |         61 | 31.0409483000 | 31.3784704000 |    1.480587078948432 |
+----+------------------+------------+---------------+---------------+----------------------+

select MIN( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities;

返回如下结果：

select * , ( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities

+----+------------------+------------+---------------+---------------+----------------------+
| id | name             | country_id | lat           | lng           | min_dis              |
+----+------------------+------------+---------------+---------------+----------------------+
|  1 | Cairo            |         61 | 30.0444196000 | 31.2357116000 |   1.7642055948326205 |
|  2 | Alexandria       |         61 | 31.2000924000 | 29.9187387000 | 0.012723270627083274 |
|  3 | Tanta            |         61 | 30.7865086000 | 31.0003757000 |   1.1707286078440424 |
|  7 | North Coast      |         61 | 28.0488161000 | 34.4371483000 |    5.521208240105792 |
|  8 | Marsa Matruh     |         61 | 31.3543445000 | 27.2373159000 |    2.673858069059212 |
|  9 | Hurghada         |         61 | 27.2578957000 | 33.8116067000 |    5.552079415567052 |
| 10 | Ismailia         |         61 | 30.5964923000 | 32.2714587000 |   2.4415049795085366 |
| 11 | Ain ElSokhna     |         61 | 29.5927778000 | 32.3416667000 |    2.920079170546876 |
| 12 | El Mansoura      |         61 | 31.0409483000 | 31.3784704000 |    1.480587078948432 |
+----+------------------+------------+---------------+---------------+----------------------+

select MIN( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities;

正如您在结果“Alexandria”city中所看到的，ID=2的城市距离用户的位置最近

当我尝试这样的事情时：

select * , ( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities

+----+------------------+------------+---------------+---------------+----------------------+
| id | name             | country_id | lat           | lng           | min_dis              |
+----+------------------+------------+---------------+---------------+----------------------+
|  1 | Cairo            |         61 | 30.0444196000 | 31.2357116000 |   1.7642055948326205 |
|  2 | Alexandria       |         61 | 31.2000924000 | 29.9187387000 | 0.012723270627083274 |
|  3 | Tanta            |         61 | 30.7865086000 | 31.0003757000 |   1.1707286078440424 |
|  7 | North Coast      |         61 | 28.0488161000 | 34.4371483000 |    5.521208240105792 |
|  8 | Marsa Matruh     |         61 | 31.3543445000 | 27.2373159000 |    2.673858069059212 |
|  9 | Hurghada         |         61 | 27.2578957000 | 33.8116067000 |    5.552079415567052 |
| 10 | Ismailia         |         61 | 30.5964923000 | 32.2714587000 |   2.4415049795085366 |
| 11 | Ain ElSokhna     |         61 | 29.5927778000 | 32.3416667000 |    2.920079170546876 |
| 12 | El Mansoura      |         61 | 31.0409483000 | 31.3784704000 |    1.480587078948432 |
+----+------------------+------------+---------------+---------------+----------------------+

select MIN( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities;

我明白了：

+----------------------+
| min_dis              |
+----------------------+
| 0.012723270627083274 |
+----------------------+

+----+-------+------------+---------------+---------------+----------------------+
| id | name  | country_id | lat           | lng           | min_dis              |
+----+-------+------------+---------------+---------------+----------------------+
|  1 | Cairo |         61 | 30.0444196000 | 31.2357116000 | 0.012723270627083274 |
+----+-------+------------+---------------+---------------+----------------------+

这对我来说很有意义。。。但当我尝试这个：

select * , MIN( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities;

我明白了：

+----------------------+
| min_dis              |
+----------------------+
| 0.012723270627083274 |
+----------------------+

+----+-------+------------+---------------+---------------+----------------------+
| id | name  | country_id | lat           | lng           | min_dis              |
+----+-------+------------+---------------+---------------+----------------------+
|  1 | Cairo |         61 | 30.0444196000 | 31.2357116000 | 0.012723270627083274 |
+----+-------+------------+---------------+---------------+----------------------+

返回的min_dis正确，但城市数据不正确！！我原以为它应该归还“亚历山大”的唱片，但它没有。。 有人知道为什么吗

编辑： 这个问题对我很有用：

SELECT `cities`.* FROM `cities` WHERE ((sqrt(pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2)))= (SELECT MIN((sqrt(pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2)))) AS min_id FROM `cities` ))

并返回：

+----+------------+------------+---------------+---------------+
| id | name       | country_id | lat           | lng           |
+----+------------+------------+---------------+---------------+
|  2 | Alexandria |         61 | 31.2000924000 | 29.9187387000 |
+----+------------+------------+---------------+---------------+

我只想知道为什么这个查询不起作用

select * , MIN( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities;

试试这个：

select * , MIN( sqrt( pow(lat - 31.205 , 2) + pow(lng - 29.907 , 2) ) ) as min_dis from cities;

select * , ( sqrt( pow(lat - 30.4990 , 2) + pow(lng - 29.4435 , 2) ) ) 
                                                    as min_dis 
from cities 
order by sqrt( pow(lat - 30.4990 , 2) + pow(lng - 29.4435 , 2) ) 
limit 1

---

您只需在第一次查询中执行order by即可获得结果当您使用聚合函数时，必须使用

GROUP by

语句正确指定任何非聚合字段，否则您将得到一个未指定的结果，这通常是聚合排序规则中使用的第一条或最后一条记录

---

MIN（…）

并不意味着从表中返回最小值，而是指当前组的最小值，由前导字段和

group by

语句指定。如果您想要整个表的最小值以及其他记录，您需要使用

orderby

来获取最小值记录，就像上面的Joe G Joseph所做的那样，或者按照您所做的那样，告诉它匹配获取

MIN（…）的子查询由于没有分组字段，所以使用了整个表的

。

您的解决方案似乎有效。。但这不是我需要的。当您订购一个集合时，需要O（N logn），但获得最小值只需要O（N），因此使用订单将导致大型数据集的性能不佳。