Re: a union is always a join!

"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:pi_rl.24249$ZP4.5099_at_nlpi067.nbdc.sbc.com...

> My point has little if anything at all to do with transactions and
> concurrency control. Those belong to implementations. My point is that
> relational calculus, or any equivalent mechanism such as relational
> algebra, while necessary for describing database updates, is not
> sufficient for that purpose because it can only apply to a single
> database, not two successive databases. The mechanism of updating the
> database cannot be reduced to mere algebraic expressions, but instead to
> asserting, in the context of what has been the case, just what in the
> world is different and exactly how. Let me explain.

I finally figured out that this is where the discussion branched off into what some of us consider mysticism.

I disagree with the last sentence. In particular, the phrase "just what in the world is different" implies, if I read it correctly, that a database update has to be mapped into a "real world update". However, this mapping is not presupposed by either the relational calculus or the relational algebra. A database update is simply the difference between the ex ante state of the database and the ex post state of the database. It is not an assertion about what in the world is different, except in the eye of the beholder.

The mathematics of relational calculus and relational algebra are fully capable, AFAIK, of describing the difference between two states of a database. Received on Mon Mar 16 2009 - 14:57:33 CET