Re: Interesting article: In the Beginning: An RDBMS history

"JOG" <jog_at_cs.nott.ac.uk> wrote in message news:1144408386.087956.261510_at_z34g2000cwc.googlegroups.com...

> If one just looks at the maths of a simple example. Consider a
> relation(ship) of fruit:

> fruit = {
> { (name, apple), (colour, green), (size, medium) }
> { (name, banana), (colour, yellow), (size, large) }
> { (name, date), (colour, brown), (size, small) }
> }

> There's no getting away from the fact that because each 'tuple' is just
> a set of attribute/value pairs the order we write them down in is
> necessarily unimportant.

Curiously, you wrote the tuples in the *order* name, colour, size. When people write some names in no particular order they usually write them in alphabetical *order* and add some phrase to explain that the order is pure alphabetical :-)

>To refer to them positionally, now that this
> is an RM-relation and not a purely mathematical-relation, every single
> tuple must have a common ordering applied to it. So for the first
> 'tuple', if we say:
>
> fruit_order = {
> ( (name, apple), (colour, green) ) ,
> ( (colour, green), (size, medium) )
> }
>
> and apply an equivalent ordering to this for all rows, only then can we
> (mathematically) refer to each attribute/value by its position.

What is the relation for the above ?.

> There clearly exists no mechanism for implementing or extracting such
> orderings to an RM-relation (and if there was, how would it be affected
> by Joins? I imagine generating a partial ordering would be the simplest
> solution). Perhaps there should?

> >
> > > - given a mathematical relation is a list of elements (whose order is
> > > specified by the order of its domains), when one refers to the 'first'
> > > or 'second' element one is using knowledge external to the model
> >
> > unless you include it in the model, in which case it is part of the
> > model. A reason why it might be an advantage in a model is if you did
> > not want to fix the attribute name or want to have synonyms without
> > designating one as most important. These are both viable for a data
> > model, even if not part of the RM.

> The only way I can see to include ordinal position within the model is
> as my last paragraph. Cardinal labels are of course no different from
> any other other field name, but as a pickie you'd prefer ordinality no?

I remember that the order of the rows is also irrelevant. Why we name attributes with strings and we don't name the rows ? :-) Why not the other way around ? :-) Received on Fri Apr 07 2006 - 14:03:42 CEST