Re: On formal HAS-A definition

On 12 mei, 18:12, Tegiri Nenashi <tegirinena..._at_gmail.com> wrote:
> On May 12, 4:31 am, Erwin <e.sm..._at_myonline.be> wrote:
>
> > See for example "Applied
> > mathematics for database professionals", which builds on the work from
> > the authors' tutor, Bert De Brock.
>
> Page#, please?

Eurhm, page 1 to 330, I would say. The book consistently references the empty relation as FI (you know, the circle with the "forward slash" intersecting it), untyped.

On page 95, bottom paragraph of the section discussing "the definition of a table", we find for example :

"Even the empty set is a table. In fact, under Definition 5-1 (...), the empty set is a table over any set. ..."

(Between brackets : the choice of the term "table" here is because the intended audience of the book is, first and foremost, SQL practitioners. I'm quite sure that the authors knew what the better term is.)

> In foreword Date & Darwen say that there is only one
> empty relation (when ignoring types). This indicates that by "empty
> relation" they meant something else, I suspect R00 (aka DUM). I
> thought standard definition for an empty relation requires it to have
> empty set of tuples only (with no constraint on the header).

I suspect that the "multiple typed empty relations versus one single empty relation for all (relation) types" relates closely to whether the intent is to define a data manipulation language. D&D have that intent, so they need assignment, so they need to address/resolve the issues caused by "one single empty relation for all types". AM4DP does not have that intent (its prime intent is to define a database design specification language that has solid foundations in maths), so they don't run into things such as 'assignment' and 'compile-time type checking' and such. So they can cleanly get away with the "one single empty relation for all relation types" position.

> > There is another view of relations in that a relation does not 'have'
> > a heading as such.
>
> So how exactly is it done for an empty relation? There is no tuples!

I don't know. It is not my preferred point of view either. I suspect proponents of this point of view would address this problem by saying that the empty relation does not have any particular heading/type. Once again, I suspect whether one can get away with that position depends on the intended goal to achieve. Received on Wed May 12 2010 - 20:54:35 CEST