Re: the two questions

"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:4749bf12$0$5303$9a566e8b_at_news.aliant.net...
> David Cressey wrote:
>
>> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> news:Dj92j.77429$Um6.17027_at_newssvr12.news.prodigy.net...
>>
>>>"JOG" <jog_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;
>>
>> Do you mean "surrogate" or "synthetic" ?
>
> Actually, what he is saying is at least one attribute must remain the
> same. Whether that attribute is a surrogate is a red herring.

Not exactly. It must not only remain the same, it must be expected to remain the same. In addition, the attribute in question must also be a key.

>
> Suppose two tuples differ completely except for one attribute you know was
> assigned arbitrarily. How do you know they are really the same thing?
> After all, all observable properties have changed. The fact that an
> arbitrary number is the same doesn't really tell us much.

Sure it does. If that arbitrary number rigidly designates an individual, then it ties the two sets of properties together. Received on Mon Nov 26 2007 - 03:50:23 CET