function AddProduct($orderId, $productId, $quantity, $discount)
    {
        try
        {
            $dbh = new PDO('oci:dbname=//localhost:1521/dbwc',
                                'db_158', '63901113');
        }
        Catch(PDOException $e)
        {
            print "Error! ".$e->getMessage()."<br>";
        }
        $dbh->beginTransaction();
        $dbh->exec("LOCK TABLE nw_product
                    IN EXCLUSIVE MODE");
        $productIds = GetOrderProductIds($orderId);
        if(in_array($productId, $productIds))
        {
            $sql = "UPDATE nw_orderdetail
                    SET quantity = quantity + :quantity, discount = :discount
                    WHERE orderid = :orderId AND productid = :productId";
        }
        else
        {
            $sql = "INSERT INTO nw_orderdetail(orderid, productid, quantity, discount, unitprice)
                    VALUES(:orderId, :productId, :quantity, :discount,
                            (SELECT unitprice FROM nw_product WHERE productid = :productId))";
        }
        $sth = $dbh->prepare($sql);
        $args = array(':quantity' => $quantity, ':orderId' => $orderId,
                ':productId' => $productId, ':discount' => $discount);
        $sth->execute($args);
        $sth1 = $dbh->prepare("UPDATE nw_product
                               SET unitsinstock = unitsinstock - ?
                               WHERE productid = ?");
        $sth1->execute(array($quantity, $productid));
        $dbh->commit();
    }