根据坐标之间的距离选择位置

Question

I attempt to make a code which calculates the nearest locations for a certain given coordinate. However I got stuck on the following error:

ERROR: column "distance" does not exist  
LINE 5: `HAVING distance < 150`  
in ROOT\frontend3.php on line 16. 

Data is retrieved from pgadminIII database. All help is welcome

<?php 

include 'connection.php';

$lat = $_GET['lat'];
$lng = $_GET['lng'];

#Select inormation from Parking database
$result = pg_query($conn, "
SELECT id, ( 6371 * acos( cos( radians($lat) ) * cos( radians( lat ) ) *     cos( radians( lng ) - radians($lng) ) + sin( radians($lat) ) * sin( radians( lat ) ) ) ) 
AS distance 
FROM parking
HAVING distance < 150 
ORDER BY distance 
LIMIT 10
");
echo $result;

Answer 1

having is for when you group/aggregate stuff and need a condition after that grouping. In your case you need to use where :

SELECT   id, ( ... ) AS distance 
FROM     parking
WHERE    distance < 150 
ORDER BY distance 
LIMIT    10

Answer 2

SELECT
    id
    ,( 6371 * acos( cos( radians($lat) ) * cos( radians( lat ) ) *     cos( radians( lng ) - radians($lng) ) + sin( radians($lat) ) * sin( radians( lat ) ) ) ) AS distance 
FROM parking
WHERE
    ( 6371 * acos( cos( radians($lat) ) * cos( radians( lat ) ) *     cos( radians( lng ) - radians($lng) ) + sin( radians($lat) ) * sin( radians( lat ) ) ) ) < 150
ORDER BY distance 
LIMIT 10;

2 issues HAVING is used when using group by for aggregations and is applied after the dataset. So a where statement would be more appropriate for your. Second distance is a column alias so it won't be available in a where clause you can solve that by putting the entire calculation in the where clause. Or use a sub select.

SELECT *
FROM (
    SELECT
       id
       ,( 6371 * acos( cos( radians($lat) ) * cos( radians( lat ) ) *     cos( radians( lng ) - radians($lng) ) + sin( radians($lat) ) * sin( radians( lat ) ) ) ) AS distance 
    FROM parking
) t
WHERE
    distance < 150
ORDER BY distance 
LIMIT 10;