Bind TreeView Data

I've a table like
AgentID AgentName SpID
2         a         1
3         b         2
4         c          2
5         d         4
6         e         5
I checked example from
http://www.aspsnippets.com/Articles/Bind-and-display-hierarchical-data-with-TreeView-in-ASPNet-using-C-and-VBNet.aspx
but it is showing only 1 level in my code there might be n no. of child level like 4 is under ...