MySQLデータベースからGoogleMapsAPIv3で複数のポリゴンを描画する

google.maps.Polygon パスプロパティは、google.maps.LatLngsの配列の配列を取ります。各サブディビジョンを独自の配列として処理し、それを個別のパスとして1つのポリゴンにプッシュするか、（以下のように）それぞれに新しいポリゴンを作成する必要があります。

    var subdivision = xml.getElementsByTagName("subdivision");
    for (var i = 0; i < subdivision.length; i++) {
        arr = [];
        var coordinates = xml.documentElement.getElementsByTagName("subdivision")[i].getElementsByTagName("coord");
        for (var j=0; j < coordinates.length; j++) {
          arr.push( new google.maps.LatLng(
                parseFloat(coordinates[j].getAttribute("lat")),
                parseFloat(coordinates[j].getAttribute("lng"))
          ));
            
          bounds.extend(arr[arr.length-1])
        }
        polygons.push(new google.maps.Polygon({
            paths: arr,
            strokeColor: '#FF0000',
            strokeOpacity: 0.8,
            strokeWeight: 2,
            fillColor: '#FF0000',
            fillOpacity: 0.35
        }));
        polygons[polygons.length-1].setMap(map);
    }

作業フィドル

コードスニペット：

function initialize() {
  var mapOptions = {
    zoom: 5,
    center: new google.maps.LatLng(40, -117),
    mapTypeId: google.maps.MapTypeId.ROADMAP
  };

  var map = new google.maps.Map(document.getElementById('map-canvas'), mapOptions);
  var arr = new Array();
  var polygons = [];
  var bounds = new google.maps.LatLngBounds();

  // downloadUrl("subdivision-coordinates.php", function(data) {
  var xml = xmlParse(xmlString);
  var subdivision = xml.getElementsByTagName("subdivision");
  // alert(subdivision.length);
  for (var i = 0; i < subdivision.length; i++) {
    arr = [];
    var coordinates = xml.documentElement.getElementsByTagName("subdivision")[i].getElementsByTagName("coord");
    for (var j = 0; j < coordinates.length; j++) {
      arr.push(new google.maps.LatLng(
        parseFloat(coordinates[j].getAttribute("lat")),
        parseFloat(coordinates[j].getAttribute("lng"))
      ));

      bounds.extend(arr[arr.length - 1])
    }
    polygons.push(new google.maps.Polygon({
      paths: arr,
      strokeColor: '#FF0000',
      strokeOpacity: 0.8,
      strokeWeight: 2,
      fillColor: '#FF0000',
      fillOpacity: 0.35
    }));
    polygons[polygons.length - 1].setMap(map);
  }
  // });
  map.fitBounds(bounds);
}
var xmlString = '<subdivisions><subdivision name="Auburn Hills"><coord lat="39.00748" lng="-92.323222"/><coord lat="39.000843" lng="-92.323523"/><coord lat="39.000509" lng="-92.311592"/><coord lat="39.007513" lng="-92.311378"/><coord lat="39.00748" lng="-92.323222"/></subdivision><subdivision name="Vanderveen"><coord lat="38.994206" lng="-92.350645"/><coord lat="38.985033" lng="-92.351074"/><coord lat="38.984699" lng="-92.343092"/><coord lat="38.981163" lng="-92.342234"/><coord lat="38.984663" lng="-92.3335"/><coord lat="38.993472" lng="-92.333179"/><coord lat="38.994206" lng="-92.350645"/></subdivision><subdivisions>';

/**
 * Parses the given XML string and returns the parsed document in a
 * DOM data structure. This function will return an empty DOM node if
 * XML parsing is not supported in this browser.
 * @param {string} str XML string.
 * @return {Element|Document} DOM.
 */
function xmlParse(str) {
  if (typeof ActiveXObject != 'undefined' && typeof GetObject != 'undefined') {
    var doc = new ActiveXObject('Microsoft.XMLDOM');
    doc.loadXML(str);
    return doc;
  }

  if (typeof DOMParser != 'undefined') {
    return (new DOMParser()).parseFromString(str, 'text/xml');
  }

  return createElement('div', null);
}

google.maps.event.addDomListener(window, 'load', initialize);

#map-canvas,
body,
html {
  height: 100%;
  width: 100%;
}

<script src="http://maps.google.com/maps/api/js?key=AIzaSyCkUOdZ5y7hMm0yrcCQoCvLwzdM6M8s5qk"></script>
<div id="map-canvas"></div>