Re: Perhaps an idiotic question

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 01 Dec 2006 14:09:06 GMT
Message-ID: <6IWbh.27208$cz.412394_at_ursa-nb00s0.nbnet.nb.ca>

Gene Wirchenko wrote:

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

>>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.
>
> I do not see it. Could you please post some p-code?

Something like the following:

EE = extend (Emp where mgr# = e#)

       add bosses as relation same heading as EE {}     union

      extend (Emp where mgr# != e#)
       add bosses as (EE where EE.mgr# = mgr#)

Received on Fri Dec 01 2006 - 15:09:06 CET