Re: the two questions

From: JOG <jog_at_cs.nott.ac.uk>
Date: Tue, 27 Nov 2007 02:05:49 -0800 (PST)
Message-ID: <1e188ff4-457e-4db7-be50-706d26c575c1_at_v4g2000hsf.googlegroups.com>


On Nov 26, 7:56 pm, vldm10 <vld..._at_yahoo.com> wrote:
> On Nov 23, 10:36 pm, JOG <j..._at_cs.nott.ac.uk> wrote:
>
> > On Nov 24, 12:38 am, vldm10 <vld..._at_yahoo.com> wrote:
>
> > > Not long time ago on this NG there were few posts which involved an
> > > entity with 200+ attributes.
> > > Let all these attributes satisfy the following two conditions:
> > > 1) All these attributes are mutually independent
>
> > Then there are no functional dependencies so the entity can only be
> > identified by the collection of all its attributes - and hence you'd
> > end up with an equivalent superkey.
>
> The condition that the attributes are mutually independent is OK,
> because it is about the relations that are in BCNF or 'close' to
> BCNF.
>
> > If any of those attributes
> > "change" it would also therefore be a different entity altogether.
>
> Here we don't design database for "frozen" real world objects, here we
> design database for the changeable real world objects. "frozen" DBs
> are kind of na´ve databases, while "Temporal DBs"
> are more general and can be very complex. So "temporal DB" is model
> (should be) for the changeable entities.
>
>
>
> > > 2) Every attribute of an entity can change its value - like in
> > > "Temporal DB"
>
> > Nope, not gonna squeeze that one past. If they are all unstable, well
> > then, you are saying there is not a single attribute that is
> > consistent over the entity's lifetime?
>
> This condition is also OK. They are not all unstable. This means that
> in a row, one group of the attributes is changeable, but in another
> row another group is changeable. It is also possible that all
> attributes in one row are changeable. In Date, Darwen & Lorencos book,
> this case has the special attention. ( I didn't know that J.Leffler
> also has contribution in the book)
>
> Vl Odrljin
>
> > In that case how could you
> > ever identify it in the real world following change? Perhaps hire
> > someone to follow it down the street continually pointing at it?
> > Y'know, Its strange we don't get more of that in daily life, given the
> > popularity of OID's in IT... oh well, I guess we're stuck with the old
> > fashioned method of identifying things by looking at them.
>
> > > Now I have two questions:
>
> > > 1) How many attributes has a key of the corresponding relation?
> > > 2) How many attributes has a key of m-n relationship between the two
> > > mentioned entities?
>
> > A binary relationship, without use of a surrogate, would obviously
> > require twice the number of attributes that made up the aforementioned
> > superkey.
>
> > Hmmm, why do I get the feeling you're about to try and sell me
> > something? ;)
>
> > > Vladimir Odrljin- Hide quoted text -
>
> > - Show quoted text -

Again I fear there is confusion between conceptual and logical layers here. Unfortunately I can't entirely follow your posts Vladmir so I can't be sure - i think we have a language barrier at work. However, I would suggest looking at the work of Geach if you are really interested in modern ideas about change and relative identity. Received on Tue Nov 27 2007 - 11:05:49 CET

Original text of this message