Re: <OR> predicate?

On 27/09/2010 11:45 AM, paul c wrote:
> "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'?

After looking at it again with the help of a cup of wine, I think I should retract my second question. Maybe more could be spelled out in the two definitions that bothered me:

"... The heading Hr of r is a set of attributes (i.e., ordered pairs of the form <A,T>). By definition, no two attributes in that set contain the same attribute name A."

and

"... 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."

but being an attempt at formality, I guess it is proper that it uses as few words as possible. Don't know why I thought I was confused now that I look again because there is no reason to think the two sets can't be empty.

(Not that it matters much, but I guess this means the one tuple in TABLE_DEE is equal to the empty set.) Received on Tue Sep 28 2010 - 03:11:02 CEST