Here is another article by the same author (actually it somwhat seems to be a longer version of the same). You have to scroll through the first sections of each article to get to the "Tree's in SQL" stuff.
Part one of the article ...http://www.dbmsmag.com/9603d06.html
Then part two of the article ...http://www.dbmsmag.com/9604d06.html
and the third part ...http://www.dbmsmag.com/9605d06.html