Re: Another view on analysis and ER

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 07 Dec 2007 13:29:49 -0400
Message-ID: <4759830f$0$5265$9a566e8b@news.aliant.net>

David Cressey wrote:

> "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?

Instantiation. Received on Fri Dec 07 2007 - 11:29:49 CST