--- In firebird-support@yahoogroups.com, Alexandre Benson Smith

> You could make a self-referencing table example
>
> ID Description Parent
> 1 Root <null>
> 2 Node1 1
> 3 Node2 1
> 4 SubNode 1.1 2
> 5 SubNode 1.2 2
> 6 SubNode 2.1 3
> 7 SubNode 1.1.1 4
>
> And so on...
>
> Look here
> http://www.dbmsmag.com/9603d06.html (Joe Celko Article)

Celko has wonderful book devoted to this subject:
- Joe Celko's Trees and Hierarchies in SQL for Smarties

http://www.amazon.com/exec/obidos/ASIN/1558609202/qid=1124263248/sr=2-2/ref=pd_bbs_b_2_2/103-4503381-5023807
or http://shorl.com/fudesabradrudo

highly recomended (as is his whole series)