Re: attribute name conflicts

"paul c" <toledobythesea_at_oohay.ac> wrote in message news:vFwgi.68094$NV3.49519_at_pd7urf2no...
> Bob Badour wrote:
>> paul c wrote:
>> ...
>> Did you perhaps mistype and mean: "Does it ever make sense within a given
>> application (as opposed to within a given db) to have two different
>> *attributes with identical names* that identify different types/domains?"
>>
>> If so, I would say: Yes, absolutely. While I would expect the much more
>> common case is for similarly named attributes to have the same type, I
>> can imagine all sorts of cases where one might use the same name for
>> different types of things.
>>
>> Capacity might mean a volume in one case, a weight in another case and
>> stored energy in a third case.
>
> Thanks, yes I did mis-type and meant it the way you put it.
>
> Regarding "capacity", I think I'd prefer for an app to use the three
> different names: "volume", "weight" and "energy". My main reason
> (psychological) would be that I find it helpful to have as much
> transparency as possible (as one might gather from reading my frequent
> mis-typings) but a technical reason might be that then I wouldn't need to
> deal with "exceptions" (I presume that an expression like "A JOIN B" where
> A and B use the "capacity" attribute name for different types would, in
> the purest sense, be considered not well-formed, ie., it would not be
> strictly logical to give a result such as "empty" or "false" for such an
> expresssion.)
>

It would indeed be malformed. An attribute name denotes a role that a particular domain plays within a relation. A natural join (which is what I assume you are referring to by A JOIN B) joins relations on a common set of role-qualified domains, but Capacity in the operands A and B do not draw their values from the same domain. Assuming that A and B do not share any other attributes, the result should therefore be the product of A and B, but the role names in the product would not be unique, so the result would not be valid.

It should be noted that if attribute names are unique throughout the database schema, that is, if the scope of the name of an attribute is extended from denoting a role that a domain plays within a single relation to denoting a role that a domain plays throughout the entire database, then the expression "A JOIN B" would always be a valid expression (provided, of course, that relation schemata A and B are defined).

> p
Received on Sat Jun 30 2007 - 17:03:34 CEST