Re: What databases have taught me

Marshall <marshall.spight_at_gmail.com> wrote:
> Dmitry A. Kazakov wrote:
> >
> > My feeling is that it could turn quite hard for RA. That's why I've
> > presented it! (:-)) The feeling is based on a trivial observation that the
> > nodes of the dual graph aren't present in the original graph either as
> > nodes or as relations. You have to "invent" them (identify the regions).
>
> Every query is an "invention" in this sense. Sometimes it is a trivial
> one, when all one does it pick out some rows and/or columns. But
> you can also write expressions. Simple example: select a+b as c from R.

I don't know anything about the theoretical limits of relational algebra or SQL. However, my intuition is that Dmitry might be right about finding a planar embedding. I believe I mentioned earlier that it's neither an obvious nor a trivial problem. Aside from that, though, the reason I doubt the ability to express it in SQL is, roughly speaking, that there are several right answers, each of which is going to be some arbitrary number of tuples, and there is no obvious way to distinguish between the answers. My feeling (and I say feeling in the sense of "I don't really know") about SQL is that there's no obvious way to ask for just the first answer that becomes apparent, nor for it to assign some arbitrary "solution number" for each solution. So I wouldn't even know what results in relational form, much less how to write the query!

If I'm wrong about the above, then you can ignore my conclusion. I don't claim to have knowledge in this area.

-- 
Chris Smith - Lead Software Developer / Technical Trainer
MindIQ Corporation