Re: On formal HAS-A definition

"Mr. Scott" <do_not_reply_at_noone.com> wrote in message news:Atqdnbh1eNZ_YX7WnZ2dnUVZ_g-dnZ2d_at_giganews.com...
>
> "Tegiri Nenashi" <tegirinenashi_at_gmail.com> wrote in message
> news:0b0623a8-7a8c-476b-8de2-78c31a36ab17_at_f17g2000pra.googlegroups.com...
>> Again, I didn't research literature, but here is my shot: the HAS-A is
>> an inclusion dependency. Example:
>>
>> Dept = [DeptNo DeptName]
>> 10 Accounting
>> 20 Research
>> ;
>>
>> Emp = [DeptNo EmpName]
>> 10 King
>> 10 Smith
>> ;
>>
>> Formally:
>>
>> Emp v (Dept ^ []) < Dept v (Emp ^ []).
>>
>> I suppose HAS-A shares many unconvenient properties with set
>> membership, for example, it is not transitive. Consider
>>
>> Accounts = [EmpName Institution]
>> Smith BoFA
>> Smith WellsFargo
>> ;
>>
>> the it is not the case that "Dept HAS-A Accounts". Again, the naming
>> problem raises its ugly head: why would the first attributes be called
>> "EmpName" rather than "PersonName"?
>>
>> More important: is this correct formalization? Specifically, shouldn't
>> functional dependency
>>
>> Dept # DeptNo < Dept # DeptName
>>
>> be a part of HAS-A constraint definition?
>
> I don't think nontrivial functional dependencies have any bearing on
> whether or not there is a 'has-a' relationship; instead, it is the
> juxtaposition of attributes in a relation scheme under the convention that
> it is not just components that are significant but also tuples. If there
> is a mapping from tuples into the domain of discourse, then it follows
> that the components of a tuple map to parts of the whole that the tuple
> maps to.

One could argue that a trivial functional dependency from the superkey that consists of the entire heading of a relation to any proper subset of the heading specifies a 'has-a' relationship.

I should also point out that because 'has-a' relationships are in essence part-whole relationships, that they are transitive. For example, cars have tires; tires have rims; therefore, cars have rims. Received on Sat May 08 2010 - 15:58:09 CEST