Try this
Please use the below code to get lat and long for touch point.
CGRect r = mapView.bounds;
float longitudePercent = overlay.point.x / r.size.width;
float latitudePercent = overlay.point.y / r.size.height;
CLLocationCoordinate2D coord = mapView.centerCoordinate;
MKCoordinateSpan span = mapView.region.span;
float lat = coord.latitude + (span.latitudeDelta/2);
float lon = coord.longitude - (span.longitudeDelta/2);
Just add your rectangle's length and width in overlay.point.x and overlay.point.y and get other lat and long.
after getting all 4 lat and long make a sql query on database and get related results for that.