Re: RA with MV attributes

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"? 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 ...

-- 
Jon