Re: RA with MV attributes
Date: Wed, 17 Jan 2007 11:04:20 +0100
Message-ID: <eoksb1$54q$1_at_orkan.itea.ntnu.no>
David wrote:
> Jon Heggland wrote:
>> David wrote: >>> r1 |X| r2 (Names,Cars,Colours) >>> bill car1,car4 red >>> bill car2 green >>> john,fred car3 red >>> john,fred green >>> >>> [...] >>> For example the >>> last tuple of r1 |X| r2 above doesn't imply that John and Fred >>> don't own any cars. >> So what exactly does that last tuple mean?
>
> It would appear that this tuple only states that John and Fred exist
> and the colour Green exists. This information is redundant with
> respect to the other tuples.
"It would appear"? You are guessing?
Anyway, I assume the other tuples /do/ state more, i.e. "bill car2 green" does not merely state that Bill exists, Car2 exists and Green exists? (I'll refrain from asking what exactly the meaning of this "exists" business is.) So the tuples have different kinds of meaning? What determines this meaning? The presence or absence of empty sets? What is simple, intuitive and elegant about this?
> I don't even think the tuple should be regarded as implying John and
> Fred don't have any green cars. I would avoid using individual
> tuples to infer that a relationship doesn't exist in other tuples.
> I would say tuples can only add relationships, never remove them.
And often, tuples don't even add relationships, just confusion and redundancy, it seems ...
-- JonReceived on Wed Jan 17 2007 - 11:04:20 CET