Need to perform mathematical operation on SQL query with multiple joins on same table

Question

I'm using WordPress for custom fields, and it basically has a meta_key column and a meta_value column in wp_postmeta table which I am using to store meta_keys 'longitude' and 'latitude' with meta_values say, 24.3 and 76.2 for one post_id of type 'gym'.

Now I need to query the database from Php to find the nearby gyms from the user position within a 10km radius. I am using the following query:

                 SELECT
                 pm1.meta_value AS longitude,
                 pm2.meta_value AS latitude,
                 ACOS( SIN( RADIANS( pm2.meta_value ) ) * SIN( RADIANS( $user_lat ) ) + COS( RADIANS( pm2.meta_value ) )
                * COS( RADIANS( $user_lat )) * COS( RADIANS( pm1.meta_value ) - RADIANS( $user_lng )) ) * $earth_radius AS distance 

                 FROM `wp_postmeta` pm1
                   INNER JOIN `wp_postmeta` pm2 ON pm1.post_id = pm2.post_id

                 WHERE
                   pm1.meta_key = 'longitude'
                   AND pm2.meta_key = 'latitude'
                   AND ACOS( SIN( RADIANS( pm2.meta_value ) ) * SIN( RADIANS( $user_lat ) ) + COS( RADIANS( pm2.meta_value ) )
                * COS( RADIANS( $user_lat )) * COS( RADIANS( pm1.meta_value ) - RADIANS( $user_lng )) ) * $earth_radius < 10

where $user_lat and $user_lng are Php variables denoting user position. This isn't working. The syntax seems to be correct but the calculated distances not. I got the inspiration for the query from radius search with google maps and mysql

What am I doing wrong?

UPDATE: The above code works. I was making a very silly mistake. Above code is a great resource for people facing the same problem where longitude and latitude fields aren't clearly defined, for example in the default WordPress custom fields table layout.

Answer 1

Can't you just calculate distance between each gym and user position, and return the ones where distance <=10km?

For example:

User coordinates = (7, 2)

Gym1 coordinates = (2,4)
Gym2 coordinates = (10,3)
...

distance1 = sqrt((7-2)^2+(2-4)^2)
distance2 = sqrt((7-10)^2+(2-3)^2)
...