RE: unhappy responses

>From: Alan Ruttenberg [mailto:alanruttenberg@gmail.com]

>>>Marijke appears to be confused about keys.
>>
>> Maybe this pointer will be sufficient to resolve her confusion:
>>
>>  <http://en.wikipedia.org/wiki/Alternate_key>
>
>It is different to say that there can be more than one key property,
>than to say for any given key property a single individual can have
>more than one value. It is the latter she seems to be complaining
>about.

It's true, I cannot (in the basic relational model) have multiple values
with a single attribute name in a table of a relational database. So if I
want to have multiple email addresses with the same dataset, then I have to
either define several attribute names for the same "conceptual" attribute in
the same table, such as "email1", "email2", ..., "emailN", or, if there is
no natural upper bound for the number of email addresses, then I need a
second table. 

But this is a limitation of the relational data model. One cannot conclude
from this that it does not make sense to allow keys to be multi-value.
Conceptually, "email" is really a multivalue key, because I may have more
than one (I actually have), and each of my email addresses will uniquely
identify me as the owner of these email addresses (and provided that
everyone has an email address). I guess, FOAF people will agree.

But even if people disagree with this argumentation, I would still see no
good justification for having implicit functionality, given that it is so
simple and straight forward to explicitly make a key functional, if one
wants this. Engineering tools may also easily support this (as an option),
if there is user request. The other way around, removing functionality from
a key axiom, if one does not want it, doesn't look that straight forward to
me, though... :->

Michael

--
Dipl.-Inform. Michael Schneider
Research Scientist, Dept. Information Process Engineering (IPE)
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Email: michael.schneider@fzi.de
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