织梦如何按照软件下载次数排列

if($orderby == 'down')
{
 $sql = "select SUM(downloads) as down_num,id from dede_downloads group by id ORDER BY down_num DESC LIMIT 0,15";
 $dsql->Execute('me',$sql);
 while($drow = $dsql->GetArray() )
 {
  $arr[] = $drow['id'];
 }
 $dstr = implode(",",$arr);
 if($orwhere!='')
 {
  $orwhere .= " AND arc.id in($dstr)";
 }else{
  $orwhere .= " where arc.id in($dstr)";
 }
 $ordersql = 'ORDER BY field(arc.id,'.$dstr.')';
}