Re: the two questions
Date: Mon, 26 Nov 2007 02:38:00 GMT
Message-ID: <ckq2j.24562$JD.8797_at_newssvr21.news.prodigy.net>
"JOG" <jog_at_cs.nott.ac.uk> wrote in message
news:ba48040b-485c-483c-9e1d-5cff82845c62_at_e6g2000prf.googlegroups.com...
> On Nov 25, 7:16 am, "Brian Selzer" <br..._at_selzer-software.com> wrote:
>> "JOG" <j..._at_cs.nott.ac.uk> wrote in message
>>
>> news: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.
>>
>> 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.
> > Oh lets not do the dance all over again brian. Its got the same > gene's, whatever. Something has to be consistent for them to be the > same thing. Even if it just ends up being the fact that they share the > same path of x,y locations over time (I'd use a surrogate for that one > hey, otherwise it be a hell of a long attribute), there has to be > something. > > There just is no great big invisible pointing hand amigo. >
From a given database value D, you can arrive at a new database value D' in one of two ways: (1) by simply stating D', or (2) by stating how D must be transformed to produce D'. Either way, you still arrive at the same answer, D', but there can be a great many possible transformations from D to D'. Identifying which transformation is actually occurring makes it possible to track an individual across database transitions: this is--in effect--the great big invisible pointing hand!
>>
>> 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.
> > If it helps, why don't you view your butterfly/caterpillar as a little > "insect life" entity. Identify that. Then you might start to see that > T1-T6 are just different snapshots of it. But for any snapshot, to > recognize it as our "insect life" entity, we need that identifier. >
Not if T1-T6 are not just snapshots but also successive snapshots. If the identification at T1 is known, and the assertions that yield the values at T2, T3, T4, T5 and T6 are known, then the identification at any snapshot can be derived from the available information.
>>
>> >> 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
>
Received on Mon Nov 26 2007 - 03:38:00 CET