Re: Object-oriented thinking in SQL context?
Date: Thu, 18 Jun 2009 20:35:05 -0300
Message-ID: <4a3acf2d$0$23740$9a566e8b_at_news.aliant.net>
Nilone wrote:
> 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
I have more concern for his coherence than for his patience. Received on Fri Jun 19 2009 - 01:35:05 CEST