Re: a union is always a join!
Date: Wed, 18 Mar 2009 11:55:10 GMT
"Brian Selzer" <brian_at_selzer-software.com> wrote in message news:Sy0wl.22503
> I disagree. Your argument rests on the premise that keys permanently
> identify things in the universe of discourse. But by definition, all that
> is required is that no two tuples in the same relation in the same
> database have the same key components. What that means is that what
> identifies something in the universe of discourse at t' could identify
> something totally different at t''.
> In other words, just because keys are the same at different times doesn't
> necessarily mean that they map to the same thing in the universe of
> discourse, and similarly, just because keys are different at different
> times doesn't necessarily mean that they don't map to the same thing in
> the universe of discourse.
This is why I brought Heraclitus into the discussion. Incidentally, I disagree with both you and Heraclitus.
Without immutable identifiers, the universe of discourse cannot be
adequately described by data.
If the universe of discourse cannot be adequately described by data, then the connection between
the universe of discourse and any database becomes haphazard, at best.
Every universe of discourse that I've dealt with in connection with
databases has held to
the assumption that identifiers are immutable. This, as you said, is orthogonal to the
Relational Model, but it's an important feature of a universe of discourse, from the point of view of the semantics of the data.
The question of whether the universe of discourse is a description of a part
the real world, on the one hand, or an illusion on the other hand, is a question that I leave to mystics and thinkers like Heraclitus. Received on Wed Mar 18 2009 - 12:55:10 CET