SQL Trees

Ok, so it's been bugging me for ages, I'm just going to start compiling all the SQL tree structure knowledge here. I might have started this in a previous post, and will try to find that, if I did, and aggregate the posts sometime...

• http://intelligent-enterprise.informationweek.com/001020/celko.jhtml
• http://oreilly.com/catalog/sqlpr/chapter/ch01.pdf
• http://www.amazon.co.uk/SQL-Pocket-Guide-Jonathan-Gennick/dp/0596005121
• http://stackoverflow.com/questions/577659/real-world-examples-of-tree-structures