Re: Lots of Idiotic Silly Braces?
Date: Mon, 23 Jul 2007 17:39:09 GMT
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.
> 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).
> 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.
p Received on Mon Jul 23 2007 - 19:39:09 CEST