Re: <OR> predicate?

On 27/09/2010 11:28 AM, paul c wrote:
> On 27/09/2010 10:57 AM, paul c wrote:
> ...
>> (I'm asking this question even though I personally have some difficulty
>> reconciling parts of the A-algebra formal definitions, eg., on one hand,
>> the heading of R <OR> TABLE_DEE must include the heading of TABLE_DEE
>> which is the empty set...
>
> Oops, I can see one mistake already, the heading being a set union,
> can't include the empty set.

Forgot to add, from the last line of the formal definition:

"ts = tr1 union tr2...".

So in this case, tr2 above would stand for the tuple in TABLE_DEE. Since the heading includes no ordered pair from TABLE_DEE's heading, there is no 'corresponding' ordered triple in ts and therefore not in tr2 either. So I guess that the 'value' of tr2 must be the empty set. Still I don't remember reading anywhere that the value of the one tuple in TABLE_DEE is the empty set. If I've got this right so far, I still have one problem I can't seem to reconcile, these lines from the formal definition:

"Let tr be a tuple that conforms to Hr; i.e., tr is a set of ordered triples of the form <A,T,v>, one such triple for each attribute in Hr."

I think perhaps my original second question now becomes: how can the (one) tuple in TABLE_DEE have such an "ordered triple'? Received on Mon Sep 27 2010 - 20:45:14 CEST