Slow sql select. Two tables - php

Question

My script is running too slow when searching zipcodes within 30 miles of 90210. It was duplicating some even though there are no dups in the zipcode database too, thus the DISTINCT u.id. Besides selecting only the columns I need, any ideas on how to speed this thing up?
<?php
$zipcode = queryDB("SELECT * FROM zipcodes WHERE zipcode='$location' LIMIT 1", 'a');
$distance = 30;
$sql = "SELECT DISTINCT zipcode, ( 3959 * acos( cos( radians(".$zipcode['lat'].") ) * cos( radians( lat ) ) * cos( radians( lon ) - radians(".$zipcode['lon'].") ) + sin( radians(".$zipcode['lat'].") ) * sin( radians( lat ) ) ) ) AS distance FROM zipcodes HAVING distance <= $distance ORDER BY distance";
$zipcodes = queryDB($sql, 'r');
$sql = '';
while($row = mysqli_fetch_assoc($zipcodes)){
$sql .= "u.category='$category' AND u.zipcode='".$row['zipcode']."' AND u.zipcode = l.zipcode";
if($_GET['photos'] == 'y'){ $sql .= " AND photo_1!=''"; }
$sql .= " OR ";
}
$sql = rtrim($sql, ' OR ');
$profiles = queryDB("SELECT DISTINCT u.id, u.*, l.city, l.state, l.zipcode FROM user_accounts as u, zipcodes as l WHERE $sql ORDER BY date_added DESC", 'r');
while($row = mysqli_fetch_assoc($profiles)){
//this is where i display the profile information
} ?>
UPDATE: The biggest issue was not using a join on my user_accounts select. Here's my updated sql query.
$sql = '';
$zipcodes = new ZipCodesRange($location, $distance);
foreach ($zipcodes->zipCodes as $zipcode){
$sql .= "(u.category='$category' AND u.zipcode='".$zipcode['ZIPCODE']."'";
if($_GET['photos'] == 'y'){ $sql .= " AND photo_1!=''"; }
$sql .= ") OR ";
}
$sql = rtrim($sql, ') OR ');
$sql .= ")";
$profiles = queryDB("SELECT u.id, u.photo_1, u.name, u.business, u.date_added, l.city, l.state, l.zipcode FROM user_accounts as u INNER JOIN zipcodes as l ON u.zipcode = l.zipcode WHERE $sql ORDER BY date_added DESC", 'r');

Answer 1

Per #malias from this other question explaining the haversine formula, here is a document showing different ways to improve the speed of this sql: http://tr.scribd.com/doc/2569355/Geo-Distance-Search-with-MySQL
The main goal is to limit the zipcodes being searched so that you don't have to calculate the haversine of so many records. The suggestion was to add a WHERE clause to filter the bounds of the latitude and longitude. Table indexes can be used for this rectangular filter, then the haversine can be used to filter it as a sphere.