问如何通过在ASP.NET WebForms中选择dropdownvalue将记录添加到现有表的新的可空的列中？

EN

<asp:DropDownList ID="_Country" Width="31%" runat="server" OnSelectedIndexChanged="OnSelected">
    <asp:ListItem></asp:ListItem>
    <asp:ListItem Text="USA" Value="USA"></asp:ListItem>
    <asp:ListItem Text="Russia" Value="Russia"></asp:ListItem>

protected void Changed(object sender, EventArgs e)
{
    ...
    SqlConnection con = new SqlConnection(@"Connectionblabla.............=true;");

    SqlCommand cmd = new SqlCommand("insert  into Cities  (Country) VALUES (@Country)", con);
    cmd.Parameters.AddWithValue("Country", _country.SelectedItem.Value);

    con.Open();
    cmd.ExecuteNonQuery();
    con.Close();
}