Re: the two questions
Date: Mon, 26 Nov 2007 11:56:01 -0800 (PST)
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
> 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)
> 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
> 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