Helper

 public static string CountData(string table)
    {
        using (SqlConnection con = new SqlConnection(GetCon()))
        {
            con.Open();
            string query = @"SELECT COUNT(*) FROM " + table;
            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                return ((int)cmd.ExecuteScalar()).ToString();
            }
        }
    }

    public static bool IsExisting(string ProductID)
    {
        using (SqlConnection con = new SqlConnection(GetCon()))
        {
            con.Open();
            string query = @"SELECT ProductID FROM OrderDetails
                WHERE OrderNo=0 AND ProductID=@ProductID AND UserID=@UserID";
            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                cmd.Parameters.AddWithValue("@UserID", "1");
                cmd.Parameters.AddWithValue("@ProductID", ProductID);
                return cmd.ExecuteScalar() == null ? false : true;
            }
        }
    }
    public static bool IsExisting2(string ProductID)
    {
        using (SqlConnection con = new SqlConnection(GetCon()))
        {
            con.Open();
            string query = @"SELECT ProductID FROM InquiryDetails
                WHERE ProductID=@ProductID";
            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                cmd.Parameters.AddWithValue("@ProductID", ProductID);
                return cmd.ExecuteScalar() == null ? false : true;
            }
        }
    }

    public static double GetPrice(string ProductID)
    {
        using (SqlConnection con = new SqlConnection(GetCon()))
        {
            con.Open();
            string query = @"SELECT UnitPrice FROM Product
                WHERE ProductID=@ProductID";
            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                cmd.Parameters.AddWithValue("@ProductID", ProductID);
                return Convert.ToDouble((decimal)cmd.ExecuteScalar());
            }
        }
    }

    public static bool AvailableDiscount(string productID)
    {
        using (SqlConnection con = new SqlConnection(Helper.GetCon()))
        {
            con.Open();
            string query = @"SELECT * FROM DiscountBrands db
                INNER JOIN Product p ON db.BrandID = p.BrandID
                WHERE p.ProductID=@ProductID";
            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                cmd.Parameters.AddWithValue("@ProductID", productID);
                return cmd.ExecuteScalar() == null ? false : true;
            }
        }
    }

   public static double GetDiscount(string productID)
    {
        using (SqlConnection con = new SqlConnection(Helper.GetCon()))
        {
            con.Open();
            string query = @"SELECT d.Percentage FROM DiscountBrands db
                INNER JOIN Discounts d ON db.DiscountNo = d.DiscountNo
                INNER JOIN Product p ON db.BrandID = p.BrandID
                WHERE p.ProductID=@ProductID AND d.Status=@Status";
            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                cmd.Parameters.AddWithValue("@ProductID", productID);
                cmd.Parameters.AddWithValue("@Status", "Active");
                return cmd.ExecuteScalar() == null ? 0 : (int)cmd.ExecuteScalar();
            }
        }
    }

    public static void AddToCart(string ProductID, string Quantity)
    {
        using (SqlConnection con = new SqlConnection(GetCon()))
        {
            bool existingProduct = IsExisting(ProductID);
            int qty = int.Parse(Quantity);
            double price = GetPrice(ProductID);
            double discount = GetDiscount(ProductID) / 100;
            double discountedPrice = price * (1 - discount);

            con.Open();
            string query = "";

            if (existingProduct)
            {
                query = @"UPDATE OrderDetails SET Quantity=Quantity + @Quantity,
                    Amount=Amount + @Amount
                    WHERE OrderNo=@OrderNo AND ProductID=@ProductID";
            }
            else
            {
                query = @"INSERT INTO OrderDetails VALUES
                    (@OrderNo, @UserID, @ProductID, @Quantity, @Price,
                    @Amount)";
            }
            using (SqlCommand cmd = new SqlCommand(query, con))
            {
                cmd.Parameters.AddWithValue("@UserID", "1");
                cmd.Parameters.AddWithValue("@Quantity", Quantity);
                cmd.Parameters.AddWithValue("@ProductID", ProductID);
                cmd.Parameters.AddWithValue("@OrderNo", 0);
                if (AvailableDiscount(ProductID))
                    cmd.Parameters.AddWithValue("@Price", discountedPrice);
                else
                    cmd.Parameters.AddWithValue("@Price", price);
                if (AvailableDiscount(ProductID))
                    cmd.Parameters.AddWithValue("@Amount", discountedPrice * qty);
                else
                    cmd.Parameters.AddWithValue("@Amount", price * qty);

                cmd.ExecuteNonQuery();
            }
        }
    }