Re: Object-oriented thinking in SQL context?

On Jun 18, 9:52 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> Nilone wrote:
> > "Brian Selzer" <br..._at_selzer-software.com> wrote in message
> >news:CKf_l.42$8r.40_at_nlpi064.nbdc.sbc.com...
>
> >>"Nilone" <nil..._at_mega.co.za> wrote in message
> >>news:1245264392.410845_at_vasbyt.isdsl.net...
>
> >>>"Brian Selzer" <br..._at_selzer-software.com> wrote in message
> >>>news:D28_l.32$OF1.1_at_nlpi069.nbdc.sbc.com...
>
> >>>>"Nilone" <nil..._at_mega.co.za> wrote in message
> >>>>news:1245239158.868623_at_vasbyt.isdsl.net...
>
> >>>>>"Bob Badour" <bbad..._at_pei.sympatico.ca> wrote in message
> >>>>>news:4a2ee2f5$0$23770$9a566e8b_at_news.aliant.net...
>
> >>>>>>none Reinier Post wrote:
>
> >>>>>>>Think 'class' ~ 'relation' (table)
>
> >>>>>>But that would not only be a blunder but a great blunder.
>
> >>>>>I'd like to clarify this for anyone coming from the OO side.  If you
> >>>>>map class to relation, you're breaking the OO rule of encapsulation and
> >>>>>reducing the class to a simple aggregate type (struct).  Presumably,
> >>>>>you chose an encapsulated, polymorphic abstraction device for a reason,
> >>>>>or did you do so just because you (or somebody at your company) read
> >>>>>Lhotka's book?  Classes map to domains (types) in the relation model,
> >>>>>but be aware that subclassing is NOT subtyping.
>
> >>>>I disagree.  Classes that are reference types map to relation schemata,
> >>>>not relations, and definitely not domains.  Domains were originally
> >>>>supposed to be disjoint sets of constant symbols, but instances of a
> >>>>reference type can appear different at different times, so they are
> >>>>definitely not constants; therefore, so long as there can be reference
> >>>>types, not all types are domains.  Classes that are value types, on the
> >>>>other hand, can map loosely to domains, since each instance is the exact
> >>>>same value wherever and whenever it appears.  I say loosely because
> >>>>whenever a value type is defined with more than one attribute, it is
> >>>>closer to being a relation schema for which there is and can only ever
> >>>>be exactly one instance than being a domain, and that instance could be
> >>>>referenced directly in relational expressions.
>
> >>>>Non-simple domains, though convenient, perhaps, introduce complexity
> >>>>that is rarely, if at all, required.  Usually, the same information can
> >>>>be recorded using simple domains, thereby reducing the complexity of the
> >>>>queries used to retrieve information, and I'm a great believer in the
> >>>>keep-it-simple-stupid adage.  Moreover, non-simple domains do not
> >>>>completely eliminate the need for either nested relations or the
> >>>>introduction of surrogates.  A relation that has a relation valued or a
> >>>>tuple valued attribute is not the same thing as a nested relation,
> >>>>because each non-simple component of a tuple in a nested relation can
> >>>>"mean" different things at different times, but each element of the
> >>>>domain for a relation valued or tuple valued attribute can only "mean"
> >>>>one thing for all time.  As a consequence, flattening out a nested
> >>>>relation schema may demand the introduction of surrogates.
>
> >>>I understand and agree.  Thanks for explaining.  However, I don't
> >>>understand the part about a nested relation being different from a
> >>>relation valued or tuple valued attributed.  Specifically, what do you
> >>>mean by 'each non-simple component of a tuple in a nested relation can
> >>>"mean" different things at different times'?
>
> >>Just to be clear: a nested relation is different from a /relation/ with a
> >>relation valued or tuple valued attribute.
>
> >>The meaning, or value, of a component, is the output of the valuation
> >>function (hence its name) for the first order language term that
> >>corresponds to the component.  The valuation function maps each language
> >>term that denotes to things in the snapshot of the Universe of Discourse
> >>at the instant of interpretation.  For constant symbols, the output of the
> >>valuation function is the same thing wherever and whenever it occurs.  For
> >>a term that is a composition of symbols, the output of the valuation
> >>function can be different things at different times.  For example, "the
> >>car in the handicapped parking spot" could mean a blue Volkswagen Beetle
> >>in the morning or a black Lincoln Continental in the afternoon, or the
> >>spot may be empty during lunch, in which case "the car in the handicapped
> >>parking spot" does not denote.  For an instance of a relation-valued or
> >>tuple-valued attribute, on the other hand, the output of the valuation
> >>function must be exactly the same thing wherever and whenever it appears.
> >>By defining a domain of relations or tuples, the meanings of those
> >>relations or tuples become fixed for all time.
>
> >>In another thread, I described an example relation schema for bins in
> >>warehouses in which the entire heading is the only key.
>
> >>Bins {Warehouse, Row, Shelf, Bin}
>
> >>In the same way that two distinct sets of components can map to the same
> >>bin but just at different times and that the same set of components can
> >>map to different bins at different times, two different sets of tuples or
> >>named values that each comprise a non-simple component of a tuple can map
> >>to the same thing but just at different times and the same set of tuples
> >>or named values that comprises a non-simple component can map to different
> >>things at different times.
>
> > I think I understand.  So relation valued attributes and tuple valued
> > attributes are attributes which define a relation schema, whereas each
> > nested relation defines its own schema.  Defining the domain of an attribute
> > fixes its valuation function, and the definition of a schema defines the
> > domains of the attributes in that schema.  Does that sound about right?
>
> I think it is long past due to cite Date's _Principle of Incoherence_.

It applies. I value Brian's patience as I home in on the concepts.

Nilone Received on Fri Jun 19 2009 - 01:21:24 CEST