how can i clear session stored data from Data table

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C#

ASP.NET

Hi,

I am developing a web page in this page i am placing a order with more then one items.
here i am using a session variable to store these value and then display in datatable but now i am unable to calculation of total for different variable and unable to store these value to database.
when i storing value to database only last record saved.

Here i am using two tables
1.product_order
product_id
product_name
product_rate
product_quantity
product_amount

2.product_order detail
product_id
order_id
product_name
product_quantity
product_total

my code is

protected void Createdatatable()
{
 DataTable dt = new DataTable();
 //dt.Columns.Add("Id");
 dt.Columns.Add("Product_Name");
 dt.Columns.Add("Product_Quantity");
 dt.Columns.Add("Product_Amount");
 Session["address"] = dt;
}

protected void btnAdd_Click(object sender, EventArgs e)
{
           double a, b;
           a = double.Parse(txtqty.Text);
           b = a * double.Parse(LblRate1.Text);
           txtAmount.Text = b.ToString();

              dt = (DataTable)Session["Address"];
               dr = dt.NewRow();

               dr["Product_Name"] = DropDownList1.SelectedItem.Text;
               dr["Product_Quantity"] = txtqty.Text;
               dr["product_rate"] = LblRate1.Text;
               dr["Product_Amount"] = txtAmount.Text;
               dt.Rows.Add(dr);
               dt.AcceptChanges();
               GridView1.DataSource = dt;
               GridView1.DataBind();
               GridView1.Visible = true;
}


<pre lang="cs">}

       protected void btnSubmit_Click(object sender, EventArgs e)
       {

           grandtotal();



           try
           {
               con.Open();
               cmd = new SqlCommand("insert into product_customer(customer_name,customer_phone,customer_address)values(@customer_name,@customer_phone,@customer_address)", con);
               cmd.Parameters.Add("@customer_name", SqlDbType.NVarChar).Value = txtName.Text;
               cmd.Parameters.Add("@customer_phone", SqlDbType.NVarChar).Value = TxtPhone.Text;
               cmd.Parameters.Add("@customer_address", SqlDbType.NVarChar).Value = txtAddress.Text;
               cmd.ExecuteNonQuery();


               cmd = new SqlCommand("insert into product_order(product_name,product_quantity,product_amount,product_total_amount)values(@product_name,@product_quantity,@product_amount,@product_total_amount)", con);
               cmd.Parameters.Add("@product_name", SqlDbType.NVarChar).Value = DropDownList1.SelectedItem.Text;
               cmd.Parameters.Add("@product_quantity", SqlDbType.NVarChar).Value = txtqty.Text;
               cmd.Parameters.Add("@product_amount", SqlDbType.NVarChar).Value = txtAmount.Text.ToString();
               cmd.Parameters.Add("@product_total_amount", SqlDbType.NVarChar).Value = txtTotalAmount.Text.ToString();
               cmd.ExecuteNonQuery();



               cmd.Dispose();
           }

what is the way to store all inserted value in database.
and how we make clear clear data table.