cascading drop down lists from single table

my one table consists of fields (id, flying from, flying to). thus i have one drop down for flying from and another drop down for flying to. i'm hoping to cascade them so 2nd drop down values are dependant on the values from 1st drop down. is it possible to do this using a single table? or must i use two tables and link the id's from both? and do i write anyting in the SelectedIndexChange event? thanks...
the sqldatasource code looks like this:
Flying From:
        <asp:DropDownList ID="ddlFlyingFrom" runat="server" DataSourceID="SqlDataSource1"
            DataTextField="FlyingFrom" DataValueField="Id" AutoPostBack="True" OnSelectedIndexChanged="ddlFlyingFrom_SelectedIndexChanged">
        </asp:DropDownList>
 &nbs ...