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Re: Perhaps an idiotic question

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Thu, 30 Nov 2006 23:41:50 GMT
Message-ID: <2%Jbh.27048$cz.408998@ursa-nb00s0.nbnet.nb.ca>


Gene Wirchenko wrote:

> Bob Badour <bbadour_at_pei.sympatico.ca> wrote:
>

>>Gene Wirchenko wrote:
>>
>>>paul c <toledobythesea_at_oohay.ac> wrote:
>>>
>>>>But it perplexes me even though I admit I have no good reason for asking:
>>>>
>>>>If I declare a relation (or more properly a relvar) R to have an 
>>>>attribute A where A's type is the type of R am I declaring a fallacy or 
>>>>something that is logically possible (ignoring whether it has any use)?
>>>
>>>     I see recursion but no termination.
>>>
>>>     What *is* the use of such a declaration?
>>
>>What would be the type of a view that combines a recursion or closure 
>>with the GROUP operation?

>
> I do not know since I see recursion but no termination. If that
> is the case, the type can not declared in the first place.

Sure, it can. It just requires a forward reference.

> I can see the attribute being an FK into its own relation. The
> type then is more pointer to R than R. (That wording is not quite
> right, but I hope the concept comes through.)

Suppose you have a reference from R1 to R1. e.g. Employee relation has a Manager reference to Employee. You declare a recursive view that groups the manager references as a relation valued attribute.

Whether the recusion terminates depends on what you did with the CEO's manager reference and how you wrote the recursion. Received on Thu Nov 30 2006 - 17:41:50 CST

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