Re: On Formal IS-A definition

From: Mr. Scott <do_not_reply_at_noone.com>
Date: Thu, 6 May 2010 03:00:32 -0400
Message-ID: <F82dnQQ3gYWN9H_WnZ2dnUVZ_gudnZ2d_at_giganews.com>


"Tegiri Nenashi" <tegirinenashi_at_gmail.com> wrote in message news:0b2f71d0-34b5-4661-a8f6-21a40cdb9989_at_n37g2000prc.googlegroups.com...
> Given the two relations R and S, the R is a subtype of S or simply "R
> is an S" (was this the source of Reinier blunder?-) iff the two
> conditions hold:
>
> 1. R ^ S = R (where the ^ is natural join operation). This can be
> expressed succinctly as R < S with generalized subset constraint "<".
>
> The immediate consequence is that the attributes of S are the subset
> of attributes R (formally: R ^ [] < S ^ [] where the "[]" is the
> relation with empty set of attributes and empty set of tuples, aka
> DUM). Then, one may add second requirement that
>
> 2. Attributes of S form a key in relation R.
>
> My question is if the condition #2 is really necessary. Consider the
> two relations:
>
> Animals = [Name]
> bear
> sheep
> wolf
> ;
>
> Carnivores1 = [Name FavoritePrey]
> bear deer
> wolf sheep
> ;
>
> They satisfy both conditions so that informally we say "Carnivores1"
> IS-A "Animals".
>
> Contrast this with
>
> Animals = [Name]
> bear
> sheep
> wolf
> ;
>
> Carnivores2 = [Name Prey]
> bear deer
> wolf sheep
> wolf deer
> ;
>
> I suggest that we still have "Carnivores2" IS-A "Animals". Do you
> agree?

There is a problem here. Which scheme is better?

Employees{taxid,name,startdate} KEY{taxid}, ContractEmployees{taxid,name,startdate,enddate} KEY{taxid}, ContractEmployees[taxid,name,startdate] is a subset of

    Employees[taxid,name,startdate]

or

Employees{taxid,name,startdate} KEY{taxid}, ContractEmployees{taxid,enddate} KEY{taxid}, ContractEmployees[taxid] is a subset of Employees[taxid]

Clearly the second scheme is equivalent to the first with respect to information content, so if a ContractEmployee is an Employee in the first scheme, then it must also be in the second, but in the second scheme, the set of attributes for Employees is not a subset of the set of attributes for ContractEmployees. I think the effect of the foreign key constraint, namely, that the components of the referenced tuple are effectively 'included' with the referencing tuple, must be taken into account. Received on Thu May 06 2010 - 02:00:32 CDT

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