Re: Hierarchical query

On Jun 13, 2:25 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> Btw., if you forget about sequence order, document order and value
> comparisons for a moment, the core of XPath is actually a relatively
> neat calculus of binary relations:
> - p_1/p_2 is the concatenation of binary relations
> - p_1[p_2] is the selection of pairs in p_2 whose right-hand side
> matches a left-hand side in p_2 (semijoin anyone?)
> - p_1 union p_2 is the set union of two binary relations
> - p_1 intersect p_2 is the set intersection of two binary relations
> - p_1 except p_2 is the set difference between binary relations
>
> Did I mention Tarski already? :-) Of course we had to wait until
> XPath2.0 until the three set operations were all allowed everywhere,
> but we have them now.

Yes, and you have been credited for that (by me removing that XML banner:-).

Well, the question originated from the world of Kleene algebras (which Tarski relation algebra is an instance of), idempotent semirings, all the other language theory goodies. I was temorarily confused by the concept of the "query". Now that it it no longer the case, and I can operate back into the language theory space, what do I need XML for? Received on Thu Jun 14 2007 - 00:08:43 CEST