Question

I have a Zip Code MySQL database that has the lat/lng of each zip code. How do I go about finding the center zip code for each state? I would be doing this in PHP.

Answer 1

Kludge it. Make all the states rectangles.

SELECT
  state,
  (MAX(lat)-MIN(lat))/2 + MIN(lat) 'center_lat',
  (MAX(lng)-MIN(lng))/2 + MIN(lng) 'center_lng'
FROM table
GROUP BY state

It’s not perfect, but unless you like complex math it’s the best of a bad situation.

edit: I just re-read the question, realized it’s not asking for the center lat/lng, but center zip. Having retrieved a list of the approximate center lat/lng for each state you can loop through and find the nearest ZIP to each.

SELECT
  zip,
  ABS(lat-$center_lat) + ABS(lng-$center_lng) 'diff'
FROM table
WHERE state = $state
ORDER BY diff ASC
LIMIT 1