Re: Another view on analysis and ER
Date: Fri, 07 Dec 2007 19:12:48 GMT
Message-ID: <QWg6j.161$W27.6_at_trndny09>
"Jan Hidders" <hidders_at_gmail.com> wrote in message
news:b0c48336-ae41-4041-b1c6-c55f4ed69cdc_at_s8g2000prg.googlegroups.com...
> On 7 dec, 17:12, "David Cressey" <cresse..._at_verizon.net> wrote:
> > "Jan Hidders" <hidd..._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.
>
> Indeed. Both the RM and ER modelling are ultimately based on
> predicates. That is not where the essential difference lies. Also not
> in the fact that you can draw a diagram, because this can be done in
> both cases.
>
> > What's the difference between a predicate and a proposition?
>
> My usual explanation is something like the following. A proposition is
> a specific statement that is either true or false. A predicate is a
> proposition where zero or more references have been left open such as
> "X is having a conversation with Y". If you instantiate the predicate,
> i.e., you fill in the X and Y with well understood references, like in
> "Jan is having a conversation with David", then it becomes a
> proposition. Roughly speaking you could say that the relationship
> between propositions and predicates is like the relationship between
> entities and entity types, or between objects and object classes.
>
Could they also be the difference between a relvar's intension and its extension, as I read in here?
> Somehow I suspect that what you actually wanted to hear was "they are
> the same". :-)
>
No, actually what I wanted to hear was "the same as the difference between the two Wikipedia definitions." But I will be content if I can learn the truth, regardless of what I wanted.
> -- Jan Hidders
Received on Fri Dec 07 2007 - 20:12:48 CET