无法理解在mysql查询中使用聚合函数的结果

Question

I have a table in my DB called cities, with id, name , country_id, lat(latitude) and lng(longitude). In my application I get the user's location, then try to find out which city is closest to the user's location so at first I did something like this:

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

which returned results like this:

+----+------------------+------------+---------------+---------------+----------------------+
| 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 |
+----+------------------+------------+---------------+---------------+----------------------+

as you can see in the results "Alexandria" city, with ID=2 is the closest to the user's location.

And when I try something like this:

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

I get this:

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

which makes sense to me... but when I try this:

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

I get this:

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

the min_dis returned is correct, but the city data is not!! I thought it should have returned the record of "Alexandria" but it didn't .. Does anyone know why??

EDIT: this query worked for me:

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` ))

and returned:

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

All I just want is to understand why didn't this query work?

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

Answer 1

try this:

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

You just have to do an order by in your first query to get the result

Answer 2

When you use aggregate functions, any non-aggregate fields have to be correctly specified with the GROUP BY statement, otherwise you'll end up with an unspecified result, which is usually the first or last record used in the aggregate collation.

MIN(...) does not mean return the minimum from the table, it means the minimum for the current group, which is specified by the leading fields and the GROUP BY statement. If you want the minimum for the whole table, with the other records, you do need to either use an ORDER BY to get the lowest record, as Joe G Joseph did above, or do as you have done and tell it to match a subquery that gets the MIN(...) of the whole table via virtue of having no grouping fields.