Re: Lots of Idiotic Silly Braces?

From: Brian Selzer <>
Date: Mon, 23 Jul 2007 20:24:28 GMT
Message-ID: <088pi.10373$>

"paul c" <> wrote in message news:1J5pi.34$fJ5.31_at_pd7urf1no...
> Brian Selzer wrote:
> ...
>> That's true. It is precisely the fact that each relation value for an
>> rva has the same heading that caused me to question whether an rva can be
>> a key or the only key on a base relation. This is due to the fact that a
>> relation schema with a particular heading can only have one value in a
>> given database value. ...
> No, that is not a fact. I don't object to somebody saying that their
> particular database follows a rule to that effect, but to say such a rule
> must apply to all db's is arbitrary, willful and mystical. I say phooey
> to that.

How can a relation schema have more than one relation value at the same time? Or using Date's term, how can a relvar have two different values at the same time?

>> Now it is quite possible to have two different relation schemata with the
>> same heading, thereby having two independent relation values with the
>> same heading in a given database value, but relation schemata are named,
>> whereas rva's are not (unless of course the rva is a dependent
>> attribute).
> No. Rva's have names (at least they do in the D&D definition, for all I
> know, in SQL, they don't, but I'm not talking about SQL).

The attribute has a name, true. I misspoke. The values for an rva aren't named. The values for a relation schema take on the name of the relation schema, so they are named.

>> What I question is whether a relation value can be self-determining in
>> the context of the other relation values (in other tuples) in the same
>> relation. The lack of duality between UNGROUP and GROUP means that a
>> given value for a relation schema that has an rva as the only key may
>> determine a different relation value for the same schema in addition to
>> itself. In other words, there can be more than one relation value for
>> the same schema that means precisely the same thing.
> If rva's are admitted, clearly two different rva values do not mean
> precisely the same thing, otherwise you can throw the information
> principle out the window.

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.

> p
Received on Mon Jul 23 2007 - 22:24:28 CEST

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