Re: Constraints and Functional Dependencies

paul c wrote:

> paul c wrote:
>

>> paul c wrote:
>>
>>> Marshall wrote:
>>> 
>>>> ...
>>>> Hmmm. Can we express keyness otherwise? I can't think how.
>>>> ...
>>>
>>> ... I sometimes wonder whether an alternative concept or two, such as 
>>> a variation on D&D's GROUP/UNGROUP operators might allow definition 
>>> of keys without rename or your prime operator.  ...
>>
>> eg., if you GROUP the non-key attributes of R, I think the result will 
>> equal R in the most literal way if the remaining attributes constitute 
>> a key (maybe somebody will correct me if that's wrong).

Frankly, Paul, I haven't got a clue what you are suggesting. When you say you are grouping the non-key attributes, do you mean to create an RVA with all of the non-key attributes? Or do you mean to create an RVA of the key attributes grouped on the non-key attributes?

If you create an RVA with all of the non-key attributes grouped on a key, each relation value will have cardinality 1.

If you create an RVA with all of the key attributes grouped on the non-key attributes, the cardinality of each relation value could be anything greater than zero. Received on Sun Feb 25 2007 - 20:54:31 CET