Re: Another view on analysis and ER

From: David Cressey <cressey73_at_verizon.net>
Date: Fri, 07 Dec 2007 16:12:51 GMT
Message-ID: <7ie6j.425$1p.297_at_trndny01>


"Jan Hidders" <hidders_at_gmail.com> wrote in message news:0c5c22b8-cdb3-462c-b55d-4c63d7974001_at_a39g2000pre.googlegroups.com...
> On 7 dec, 12:17, Jon Heggland <jon.heggl..._at_ntnu.no> wrote:
> > Quoth David BL:
> >
> > > I wasn't actually intending that Location be necessary for
> > > identification of a marriage. I'll make the intensional definition
> > > clearer:-
> >
> > > married(Husband, Wife, Location) :-
> > > Husband is *currently* married to Wife
> > > and they (last) got married at Location
> >
> > > Candidate keys are { Husband } or { Wife }, enforcing monogamy
> > > integrity constraints.
> >
> > So Marriage is a relationship between a Husband and a Wife, yet it is
> > identified by either, not the combination? I thought I finally had the
> > common definition of "relationship" pegged, and then this comes along.
> >
> > I suppose I am looking for rigor where there is none, though. The
> > definition of entity---something that is identified independently of
> > other entities---is also rather half-baked. Take weak entities, for
> > instance.
>
> Allow me to make an attempt at a few definitions:
>
> Entities are things.
> Relationships are predicates.
>
> What's wrong with this picture?
>

This sounds right to me. If we can go one step further, and say that unary relationships are predicates that reference only one entity, now we have a basis for moving forward. The entire lofical level is baed on predicates.

We can say, as others have said, that relvars can be designed once the predicates are known.
And this, unlike the discussions on ER, remains silent on the subject of whether the mode of epxression has to be a diagram. Predicates can be expressed in plain English.

What's the difference between a predicate and a proposition?

> -- Jan Hidders
Received on Fri Dec 07 2007 - 17:12:51 CET

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