Re: Constraints and Functional Dependencies

Keith H Duggar wrote:
> mAsterdam wrote:
>

>>A rephrase to (i) could be:
>>
>><reference>
>>(i a)
>>   A relation R with attribute a (written as R(a)) having
>>   a as a reference into S(b)
>>   is expressed as follows:
>>
>>   forall R(a): exists S(b): a = b
>>
>>   Note that b need not be a ck to S, hence 'into', not 'to'.
>></reference>

Don't know - I thought he was just trying to be brief on the way to his main point (later in his original post, he gave his ck definition.)

Regarding "(in fact the only key)" I don't think this is so, since the empty set of attributes could also be a key, aka constraint if the language in use allowed its expression. If that constraint were in place "b" would also be a key, whether it contained one attribute or many, lucky thing too, otherwise the RI interpretation that assumes ck's wouldn't be possible when the referenced table had only one row.

p Received on Sun Feb 25 2007 - 18:26:02 CET