Re: Functional Dependencies > Uniqueness Constraints
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 30 Aug 2006 19:30:16 GMT
Message-ID: <cHlJg.6237$9u.73262_at_ursa-nb00s0.nbnet.nb.ca>
>> paul c wrote:
>>
>>> Marshall wrote:
>>>
>>>> ... (And of course there must be a rule that
>>>> says every base table must have at least one functional
>>>> dependency in which the union of the determinant set
>>>> and the dependent set equals the set of attributes. (This
>>>> restriction is sufficient to ensure every base table is a
>>>> relation; is it necessary?))
>>>> ...
>>>
>>> I would say not necessary. If a table is a representation of a
>>> relation, then I`d think that even if no rule is stated, by
>>> definition the union of the attributes is a CK, eg., if there is no
>>> stated determinant set, all the attributes are in the dependent set.
>>> I can`t think why one would want to state this, shouldn`t a dbms
>>> assume itÉ
>>
>> I think you have determinant and dependent reversed. The attributes of
>> a candidate key are the determinant set, and the remaining attributes
>> are each dependent attributes. Thus, if no other key is specified, all
>> attributes are in the determinant set and the set of dependent
>> attributes is empty.
>>
>> What Marshall stated is an invariant of every relation for every
>> candidate key. In fact, it seems to me Marshall's statement is just a
>> restatement of candidate keys, but there could be subtleties I miss.
Date: Wed, 30 Aug 2006 19:30:16 GMT
Message-ID: <cHlJg.6237$9u.73262_at_ursa-nb00s0.nbnet.nb.ca>
paul c wrote:
> Bob Badour wrote: >
>> paul c wrote:
>>
>>> Marshall wrote:
>>>
>>>> ... (And of course there must be a rule that
>>>> says every base table must have at least one functional
>>>> dependency in which the union of the determinant set
>>>> and the dependent set equals the set of attributes. (This
>>>> restriction is sufficient to ensure every base table is a
>>>> relation; is it necessary?))
>>>> ...
>>>
>>> I would say not necessary. If a table is a representation of a
>>> relation, then I`d think that even if no rule is stated, by
>>> definition the union of the attributes is a CK, eg., if there is no
>>> stated determinant set, all the attributes are in the dependent set.
>>> I can`t think why one would want to state this, shouldn`t a dbms
>>> assume itÉ
>>
>> I think you have determinant and dependent reversed. The attributes of
>> a candidate key are the determinant set, and the remaining attributes
>> are each dependent attributes. Thus, if no other key is specified, all
>> attributes are in the determinant set and the set of dependent
>> attributes is empty.
>>
>> What Marshall stated is an invariant of every relation for every
>> candidate key. In fact, it seems to me Marshall's statement is just a
>> restatement of candidate keys, but there could be subtleties I miss.
> > Right, I did reverse them, in the course of trying to use Marshall's > lingo. Still if the dependent set stood for an RVA, maybe the reversal > would be true in a sort-of way.
No. In that context, the relation value is no different from any other value.
[snipped the part I found confusing] Received on Wed Aug 30 2006 - 21:30:16 CEST