I'm playing with tree structures in SQL. It's kinda cool and definitely a big hack.
Anyway, there are a few things in our database that are more hierarchal then they are relational. The problem I'm working with is accounting, as in, "accounts > owners equity > expense accounts > rent > shop rent"
Since when does Expense become a subcategory of Equity? :)
Btw, trees have been discussed so many times. Peruse the archive and you'll find many bits of wisdom...
As for meself, currently I tend to use MP (materialized path). It's space expensive but most of my trees are not very deep and MP makes most queries easy.
After all, NS (nested set) is a special form of MP anyway.
-- dave
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