Query Database trouble

Hey,

I’m running a site that has a sections of post, which use custom fields – allowing me to add reference numbers…

Then I have an ecommerce system using those reference numbers, so on my posts I need to say something like “To do this, you need these:”

Then list the name, price and quantity…

So far I have it reading the DB for the custom fields…

$key="Linked Products";
    $idcode= get_post_meta($post->ID, $key, true);
    $array = explode(",",$idcode);
  	$products = array_count_values($array);

    foreach ($products as $key => $value) {
    $sql = "SELECT product_id FROM wordpdem_wpsc_productmeta WHERE meta_value = '$key'";
    $productid = $wpdb->get_results($sql, ARRAY_N);

    	echo $sql . "<br>";
    }

The sql echo works 100%…

So now I add

while($row = mysql_fetch_array($productid)) {
			echo $row['product_id'];
		}

and I get a warning about Warning: mysql_fetch_row(): supplied argument is not a valid MySQL result resource in XXXXXwp-content\themes\xx\comments.php on line 18

Any Ideas?