Re: Another view on analysis and ER
Date: Fri, 7 Dec 2007 10:17:50 -0800 (PST)
Message-ID: <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.
> 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.
Somehow I suspect that what you actually wanted to hear was "they are the same". :-)
- Jan Hidders