Re: constraints in algebra instead of calculus

"paul c" <toledobythesea_at_oohay.ac> wrote in message news:dX84i.201343$6m4.74192_at_pd7urf1no...
> Marshall wrote:
>> Okay, a while back we were talking about writing constraints
>> in a language with aspects of the relational calculus, specifically
>> the existential and universal quantifiers. The point was made
>> that that's unnecessary; the calculus is no more expressive
>> than the algebra.
>>
>> So it ought to be possible to write any constraint from the
>> calculus in the algebra.
>>
>> Well, I'm having a hard time figuring out how to do it. Can
>> anyone help?
>>
>> How does one write a functional dependency in the algebra?
>> A foreign key?
>>
>>
>> Marshall
>>
>
> I thought that one was easy, if FK is the set of 'foreign key' attributes
> and A is the 'referencing table' and using an op like D&D <AND>, then it's
> something like A{FK} <AND> B{FK} = A{FK}. (I like this one because it's
> particularly easy to implement.)
>

Very slick. One point, though. The constraint

A{FK} <AND> B{FK} = A{FK}

Is not sufficient. Also required is the constraint

COUNT(B{FK}) = COUNT(B) Received on Mon May 21 2007 - 06:47:36 CEST