$col = "dtb_products_class.product_id"; //カラム
$from = "dtb_products_class INNER JOIN dtb_products ON dtb_products_class.product_id = dtb_products.product_id";
$where = "dtb_products_class.product_code = ? AND dtb_products_class.del_flg = 0 "; 
$arrWhereVal = array($product_code);
$res = $objQuery->get($col, $from, $where,$arrWhereVal);

echo $res;

の
$res = $objQuery->get($col, $from, $where,$arrWhereVal);
です。