Re: On Formal IS-A definition

"Tegiri Nenashi" <tegirinenashi_at_gmail.com> wrote in message news:90e167d3-4ae4-451c-9f3d-1e2c3e19c9f2_at_n15g2000yqf.googlegroups.com... On May 5, 11:00 pm, "Mr. Scott" <do_not_re..._at_noone.com> wrote:
> "Tegiri Nenashi" <tegirinena..._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.

<<QUOTE
I'd suggest different naming in the second case. The relation with attributes {taxid,enddate} is called Terminations, and a Termination IS not-An Employee. ContractEmployees relation is a view, which is a join of Employees with Terminations.
>>QUOTE
I think you missed my point. The foreign key constraint, by its very nature, has the effect of extending each row in the foreign key table to include all of the components in the corresponding row in the primary key table. There can't be a row in the foreign key table unless there is a corresponding row in the primary key table. Received on Fri May 07 2010 - 06:02:01 CEST