Re: more closed-world chatter

"paul c" <toledobythesea_at_oohay.ac> wrote in message news:RRR%h.166745$aG1.31002_at_pd7urf3no...
> Brian Selzer wrote:
> ...
>> I really don't understand what all the confusion is: perhaps it is caused
>> by conflating domains with types. A domain is simply a named set of
>> values that is specified or enumerated as part of the schema. ...
>
> Says who? (eg. who else?)
>

That is evident from Codd's first paper on the subject. I quote: "The term relation is used here in its accepted mathematical sense. Given sets S1, S2, ..., Sn (not necessarily distinct), R is a relation on these n sets if it is a set of n-tuples each of which has its first element from S1, its second element from S2, and so on. We shall refer to Si as the jth /domain/ of R." Then in a subnote, "More consisely, R is a subset of the Cartesian product S1 x S2 x ... x Sn."

> ... A type, on the other
>> hand, describes a class of values, focusing not on set membership, but
>> rather on similarities among the values--common properties, if you will.
>
> Says who? (eg. who else?)

Here's a general article on type theory: http://en.wikipedia.org/wiki/Theory_of_Types

>
> Why doesn't a domain have properties?

I'm not sure if I understand what you're driving at. A domain is a set. Any set. Each element of a domain has a type, but in general, the elements need not have the same type.

Let me put it another way. The universal set can be classified into a heirarchy of sets where each set in the heirarchy contains elements or sets of elements with a specified set of properties (the type) such that each element in the universal set belongs to one and only one set in the heirarchy (the one with the most specific type), or the universal set can be organized into an arbitrary set of domains with no restriction on the number of domains to which an element can belong.

One additional note: In RT, domains are named. Naming adds context to the values contained within a domain. So, for example, the value represented by the symbol 2 in the HOURS domain is the same element from the universal set as the value represented by the symbol 7200 in the SECONDS domain. Of course, this can not be determined unless the relationship between HOURS and SECONDS is specified as part of the schema. Naming can also be thought of as a form of classification, but one not as rigid as that in a type heirarchy, nor one that lends itself to interactions between values belonging to different domains.

>
> Is this the pot calling the kettle black? I've no objection to
> programming terms being conflated with db terms, if that motive is stated.
>

I used the terms class and property in the mathematical sense, not the OO sense.

> p
Received on Tue May 08 2007 - 10:58:24 CEST