Como obter o nome da coluna cujo valor não é nulo no mysql

$q="show columns from orders";
$res=mysql_query($q) or die(mysql_error());
$arr_field=array();
while($row=mysql_fetch_object($res)){
    $field=$row->Field;
    $q1="select ".$field." from orders where ".$field."!=0"; //if string then '0'
    $res1=mysql_query($q1) or die(mysql_error());
    if(mysql_num_rows($res1)>0){
        $arr_field[]=$field;
    }
}
$q="select ";
foreach($arr_field as $field){
    $q.=$field.",";
}
$q=rtrim($q,",");
$q.=" from orders";
$res=mysql_query($q) or die(mysql_error());
while($row=mysql_fetch_object($res)){
    foreach($arr_field as $field){
        print($field."==".$row->$field."<br/>");
    }
}