Re: the two questions

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 naïve 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 -
Received on Mon Nov 26 2007 - 20:56:01 CET