Re: Lots of Idiotic Silly Braces?

From: paul c <>
Date: Mon, 23 Jul 2007 20:46:00 GMT
Message-ID: <cs8pi.221$fJ5.172_at_pd7urf1no>

Brian Selzer wrote:

> How can a relation schema have more than one relation value at the same
> time? ...

With different names referring to the schema, whereas there is no such thing as two values for the same relation.

> ... Or using Date's term, how can a relvar have two different values at
> the same time? ...

It can't. But this shows the difficulty casual language has when referring to database devices. "Time" really has nothing to do with relations but people let the word creep in because they are often assuming the database is inextricably tied up with some sequential language or other. (People who write about transactions fall into the same trap. Please don't confuse the above with a database that abstracts time with values for some application purpose.)

> I think you're making my point for me. When a relation with a dependent rva
> is UNGROUPed, there is no information loss. When a relation is GROUPed,
> forming a dependent rva, there is no information loss. This is clearly not
> the case for a relation that has an rva as the only key. I think it is of
> critical importance that there is information loss when an rva that is the
> only attribute is UNGROUPed.

If that were so, I think one would need to define a different "UNGROUP" operator. Join and Project also lose information. Why is it critical not to?

Nothing wrong with different definitions as long as they hold up as well as the ones they replace.

p Received on Mon Jul 23 2007 - 22:46:00 CEST

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