Re: the two questions
Date: Fri, 23 Nov 2007 19:36:09 -0800 (PST)
Message-ID: <8505d954-cdc3-4bf8-9107-b307563be0e8_at_r60g2000hsc.googlegroups.com>
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. If any of those attributes "change" it would also therefore be a different entity altogether.
> 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? 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.
>
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? ;)
>
> 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?
> Vladimir Odrljin
Received on Sat Nov 24 2007 - 04:36:09 CET