Re: Relation Schemata vs. Relation Variables

From: JOG <jog_at_cs.nott.ac.uk>
Date: 22 Aug 2006 09:18:34 -0700
Message-ID: <1156263514.613028.38210@i42g2000cwa.googlegroups.com>

JOG wrote:
> Brian Selzer wrote:
> > "JOG" <jog_at_cs.nott.ac.uk> wrote in message
> > news:1156107137.284318.326750_at_m73g2000cwd.googlegroups.com...
> > > Brian Selzer wrote:
> > >> In the context of an update, the predicate of a database along with the
> > >> current database state determines the set of all /possible states/ that
> > >> can
> > >> become current. Integrity rules, which are implicitly or explicity
> > >> specified as part of the database predicate, can be classified as either
> > >> state constraints or transition constraints. State constraints define
> > >> the
> > >> set of all consistent database states; transition constraints determine
> > >> whether or not a state change should be allowed. Given a set of
> > >> consistent
> > >> database states and the current state, one can derive a set of
> > >> transitions,
> > >> each containing what is different on a tuple by tuple basis between the
> > >> current state and a proposed state (any one of the consistent states). A
> > >> transition can be defined as a set of triples (r, t, t') where r is the
> > >> name
> > >> of a relation, t is a tuple from the current state, and t' is a tuple
> > >> from
> > >> the proposed state.
> > >
> > > <argggghhhh/>.Sorry brian, but this still isn't right. It is illogical
> > > to talk about the transition of a tuple from one value to another, as
> > > though they were entities from the real world themselves. Look, say
> > > mathematically you are talking about a relation composed of three
> > > tuples:
> > >
> > > R := {x, y, z}
> > >
> > > x, y and z are not variables! They are aliases for values. I can't
> > > compare x, y an z with their future selves - they only have one value,
> > > today, tomorrow, for evermore.
> > >
> >
> > I think you're confusing attributes with tuples. Even if you're not, I
> > agree: tuples are values. If they *can* correspond, it is in the mind of
> > the designer of the database who defined the transition constraint, and thus
> > the fact that they *do* correspond must be conveyed by the user during the
> > update.

> I am not confusing anything. If you agree tuples are not variables,
> then you agree that tuples cannot 'change'. And by that one is saying
> that they /cannot/ have a transition. That's the logic, and its
> unavoidable - how can you argue against it?

You have still not explained how you intend to go against this logic. Received on Tue Aug 22 2006 - 11:18:34 CDT