Cascading DropDownList

This code how to manupulate cascading droplist.Cascading dropdownlist means dependent dropDownLists where one DropDownList is dependent on other dropDownList.
Programmer need to many occasions where bind dropdownlist on selection of another dropdownlist.

Steps:
Take three dropdownlist in aspx page
1.ddlCountry- List of Countries
2.ddlState - list of states
2.ddlCity - List of Cities

if (!Page.IsPostBack)
{
            CountryBind();
}
protected void CountryBind()
    {
        string sqlQuery = "Select * from Country;

        using (SqlConnection conn = new SqlConnection(conString))
        {
            conn.Open();
            DataSet ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(sqlQuery, conn);
            da.Fill(ds);
            ddlCountry.DataSource = ds;
            ddlCountry.DataBind();
            conn.Close();
        }
    }
    protected void StateBind()
    {
        string sqlQuery = "SELECT * FROM State " + " WHERE (CountryId = " + Convert.ToInt32(ddlCountry.SelectedValue) + " )";

        using (SqlConnection conn = new SqlConnection(conString))
        {
            conn.Open();
            DataSet ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(sqlQuery, conn);
            da.Fill(ds);
            ddlState.DataSource = ds;
            ddlState.DataBind();
            conn.Close();
        }
    }
    protected void CityBind()
    {
        string sqlQuery = "SELECT * FROM City " + " WHERE (StateId = " + Convert.ToInt32(ddlState.SelectedValue) + " )";

        using (SqlConnection conn = new SqlConnection(conString))
        {
            conn.Open();
            DataSet ds = new DataSet();
            SqlDataAdapter da = new SqlDataAdapter(sqlQuery, conn);
            da.Fill(ds);
            ddlCity.DataSource = ds;
            ddlCity.DataBind();
            conn.Close();
        }
    }

    protected void ddlCountry_SelectedIndexChanged(object sender, EventArgs e)
    {
        StateBind();
    }
    protected void ddlState_SelectedIndexChanged(object sender, EventArgs e)
    {
        CityBind();
    }

<asp:DropDownList ID="ddlCountry" runat="server" AutoPostBack="True" DataTextField="CountryName"
  DataValueField="CountryId"  OnSelectedIndexChanged="ddlCountry_SelectedIndexChanged"> 
  <asp:ListItem Value="0" Text="<Select Country> "> </asp:ListItem> 
</asp:DropDownList> 

<asp:DropDownList ID="ddlState" runat="server" AutoPostBack="True" DataTextField="StateName"
 DataValueField="StateId" OnSelectedIndexChanged="ddlState_SelectedIndexChanged"> 
</asp:DropDownList> 

<asp:DropDownList ID="ddlCity" runat="server" DataTextField="CityName" DataValueField="CityId"> 
</asp:DropDownList>