Theory question: Node trees and SQL.

I posted a question a few months ago asking how to use trees stored in
SQL using Joe Celko's "nested sets" model and re-create the tree in HTML
(and got a very quick response - thanks guys).

I'd like to ask what in your opinion is the best way to store trees in
SQL for use with XSL translation. At the moment, using the nested set
model, my XML is similar to the following:

<departments>
    <department name="Stationery" left="1" right="2" />
    <department name="Misc" left="3" right="6" />
    <department name="Haberdashery" left="4" right="5" />
</departments>

Which should produce:
   Stationery
   - Misc
   Haberdashery

While it works fine for the output, the current storage method is
volatile, and is causing us problems in manipulation.

Can anyone suggest better techniques (other than using a native XML
database)?

Thanks,
Phill