Re: Transitive Closure and Relational Division

Vadim Tropashko wrote:
...
> In a nested sets model we do hierarchical query via relational
> division. "find all the
> ancestors of node 2" is done in 2 steps:
>
> 1. Find all the elements of the set at node 2. This would be
> pi(elem) sigma(set=2) which evaluates into
>
> elem
> ----
> 2
> 4
>
> Here is is naturally to ask: "Aren't you selecting and projecting on
> the wrong columns? It
> appears as if you look for all the DESCENDANTS of the node 2!". Hang on
> to step 2.
>
> 2. Divide the relation T(set,elem) by this relation, that is find all
> the sets that contain
>
> set {2,4}.

TCRelation / ancestors = descendants?
TCRelation / descendants = ancestors? Received on Thu Dec 21 2006 - 22:44:59 CET