dede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.')';
 }