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Re: Functional Dependencies > Uniqueness Constraints

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 30 Aug 2006 18:15:16 GMT
Message-ID: <UAkJg.6205$9u.72821_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.
Received on Wed Aug 30 2006 - 20:15:16 CEST












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