Re: Guessing?

"paul c" <toledobysea_at_ac.ooyah> wrote in message news:sfm0k.179265$rd2.156266_at_pd7urf3no...
> Brian Selzer wrote:
> ...
>>> That seems to be part of an argument about something else.
>>>
>>
>> Indeed. It is not about unions but rather about strong orthogonality,
>> which is what I thought we were discussing.
>
>
> Maybe you did, but the Codd quote in the OP was about what some see as one
> of the the view updating problems, which is about whether a dbms can
> perform certain updates in a manner that is consistent with how it
> replaces base relations (I'd say replace is a far more precise word in
> this case). POOD, which is a proposed solution, was mentioned, but it
> carries other baggage having to do with human preferences and, as usual,
> gets attacked on grounds that don't have to do with the original problem.
>
>
> When the relations in a union view expression are union-compatible
> (so-called), I wish somebody could show me how any disjunction is involed
> in the tuples of the union view. Many people say that the predicate must
> be disjunctive, but I'd say that the machinery of the RM has completely
> discounted that.

I don't see how that could be. If you start out with two base relations with the same heading, then each of those relations has its own distinct predicate. The predicate of a view that is the result of the union of the two base relations is obviously just a disjunction of the predicates of the base relations, and each tuple in the view originated from either or both of the base relations, and thus satisfies at least one but possibly both of the predicates of the base relations. Received on Sun Jun 01 2008 - 05:35:16 CEST