Display all Items in Child DropDownList when ALL is selected in Parentt DropDownlist

 protected void DDLzone1_SelectedIndexChanged(object sender, EventArgs e)
    {
        BindDistrict();

    }

    private void BindDistrict()
    {
        ddldistric1.Items.Clear();
        ddldistric1.Items.Add(new ListItem("all", ""));
        ddldistric1.AppendDataBoundItems = true;

        SqlCommand _cmd = new SqlCommand("selectdistrict", _cn);
        _cmd.CommandType = CommandType.StoredProcedure;
        _cmd.Parameters.AddWithValue("@RegionID", DDLzone1.SelectedItem.Value);

        _cn.Open();
        ddldistric1.DataSource = _cmd.ExecuteReader();
        ddldistric1.DataTextField = "District";
        ddldistric1.DataValueField = "ID";
        ddldistric1.DataBind();


        _cn.Close();

    }
 

ALTER procedure  [dbo].[selectdistrict]

@RegionID varchar(5)
as
begin
select ID,District
from District
where RegionID=@RegionID or @RegionID='all'

here when i click on ALL item from ddlzone1  it just "ALL" item in ddldistrict

i want when i select "ALL" item from ddlzone1 in ddldistrict14 show all data from database