Re: the two questions
Date: Sun, 25 Nov 2007 07:16:51 GMT
"JOG" <jog_at_cs.nott.ac.uk> wrote in message
> 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.
It cannot be determined whether two representations from two distinct points in time refer to the same individual--even if all of the attribute values are identical--unless, of course, one of the attributes is a surrogate; therefore, your statement that it would be a different entity altogether is unfounded.
>> 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.
How else could you determine if the butterfly that you're looking at now is the same individual as the catepillar that you were looking at several months ago.
I think that it is most important that if you can identify an individual at multiple points in time, for example, T1 and T6, then it should be possible to track that invidual through time, noting the changes that occur at each transition, so that at T6, the individual identified /is/ the individual that had been tracked. It should be obvious that if prime attributes can be the target of an update, then it is not necessarily the case that the set of attribute values that identifies an individual at T1 identifies the same individual at T6. It may be that the set of attribute values that identifies an individual at T1 identifies a different individual at T6.
>> 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
Received on Sun Nov 25 2007 - 08:16:51 CET