how to assign id zero while selecting the default in dropdownlist

I have A drop Down list and i want that if selected nothing the id should be stored in database 0 other wise if selected something then the id of that item .??
code..

protected void Page_Load(object sender, EventArgs e)
   {
       if (!IsPostBack)
       {

           Query = @"SELECT [category_id] FROM [Category_Master]";
           ds1 = dl.fetchrecord(Query);
           ddl_parent.Items.Insert(0, new ListItem("---none---"));
           int i = 0;
           while (i < ds1.Tables[0].Rows.Count)
           {
               flag = 1;
               catname = "";
               index = 1;
               ListItem li = new ListItem();
               li.Text = catmapping(ds1.Tables[0].Rows[i]["category_id"].ToString());
               li.Value = ds1.Tables[0].Rows[i]["category_id"].ToString();
               ddl_parent.Items.Add(li);
               i = i + 1;
        }
       }
       lbl_mess.Text = "";
   }


   string catmapping(string cat_id)
   {
       while (flag == 1)
       {
           Query = "select Category_ParentId,Category_Name from Category_Master where category_id=" + cat_id + "";
           ds = dl.fetchrecord(Query);
           cat_id = ds.Tables[0].Rows[0]["Category_ParentId"].ToString();
           str = ds.Tables[0].Rows[0]["Category_Name"].ToString();
           if (index == 1)
           {
               catname = str;
           }
           else
           {
               catname = str + ">>" + catname;
           }
           if (cat_id == "0")
           {
               flag = 0;
           }
           index++;
       }

       return catname;
   }

   protected void Button1_Click(object sender, EventArgs e)
   {

       if (IsPostBack)
       {
           Query = @"INSERT INTO [RbmDatabase].[dbo].[Category_Master]
          ([Category_ParentId]
          ,[Category_Name]
          ,[Category_MetaTag]
          ,[Category_MetaTagKeywords]
          ,[Category_Description]
          ,[Category_SortOrder]
          ,[Category_Status]
          ,[Category_UpdateDate])
    VALUES ('" + dl.QoutesHandel(ddl_parent.SelectedValue.ToString()) + "','" + dl.QoutesHandel(txt_category.Text.Trim()) + "','" + dl.QoutesHandel(txt_MTdesc.Text.Trim()) + "','" + dl.QoutesHandel(txt_MTkey.Text.Trim()) + "','" + dl.QoutesHandel(txt_Desc.Text.Trim()) + "','" + dl.QoutesHandel(txt_order.Text.Trim()) + "','" + dl.QoutesHandel(ddl_status.SelectedIndex.ToString()) + "','" + String.Format("{0:yyyy-MM-dd}", DateTime.Now) + "')";
           dl.insertrecord(Query, lbl_mess);
           lbl_mess.Text = "Record Inserted  Successfull.....!";

       }

   }

Tags: ASP.NET, SQL Server