Question

I have a database of geocoded entries. I need to determine which two entries are the furthest apart from a subset of the total entries. For example, I select a list of 10 entries then, from that list, determine which two places represent the greatest distance within that list.

I cannot wrap my head around how to approach this. I’ve considered using radians even, but nothing seems to meet the requirement.

FYI, LAMP stack going here…

Answer 1

The following query will calculate the distance between all your points and return the two with the biggest distance:

SELECT coor1.longitude as lon1,
       coor1.latitude as lat1,
       coor2.longitude as lon2,
       coor2.latitude as lat2,
       (ACOS(
         COS(RADIANS(coor1.latitude))  * 
         COS(RADIANS(coor1.longitude)) *
         COS(RADIANS(coor2.latitude))  *
         COS(RADIANS(coor2.longitude)) + 
         COS(RADIANS(coor1.latitude))  *
         SIN(RADIANS(coor1.longitude)) *
         COS(RADIANS(coor2.latitude))  *
         SIN(RADIANS(coor2.longitude)) +
         SIN(RADIANS(coor1.latitude))  * 
         SIN(RADIANS(coor2.latitude))
         ) * 6378                        --- Use 3963.1 for miles
       ) 
AS DistanceKM
FROM coordinates coor1,
     coordinates coor2
WHERE NOT (coor1.longitude = coor2.longitude AND coor1.latitude = coor2.latitude)
ORDER BY DistanceKM DESC
LIMIT 1;                                 --- Only the biggest

Now I recommending doing those calculations before hand and storing the result in a separate table.