Re: Something new for the New Year (2008).

On Jan 8, 1:53 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> JOG wrote:
> > On Jan 5, 7:59 pm, Rob <rmpsf..._at_gmail.com> wrote:
>
> >>Now I know how Charlie Brown would feel when Lucy volunteers yet again
> >>to hold the football for him to kick. (My apologies if you are too
> >>young to remember Peanuts.)
>
> >>Nevertheless, I believe that the path to teaching includes tolerance
> >>and patience, so I will try to address your questions. (With some
> >>quibbles about your logic and some questions for you about the
> >>interpretation of foreign keys in predicates.)
>
> >>>There is no difference between my and Marshall's views, because they
> >>>are just a repetition of what Codd defined the relational model to
> >>>be.
>
> >>On January 1, JOG said:
> >>"Tuples in databases represent facts stated in the real world (they
> >>are not entities or objects)".
>
> >>On January 3, Marshall said:
> >>"but as far as the RM goes, it models our ideas about real-world
> >>entities and our ideas about real-world relationships in exactly the
> >>same way: as mathematical relations".
>
> >>And on January 3, JOG said:
> >>"A tuple represents a fact as an instantiation of a predicate."
>
> >>Is see a big difference here, particularly because mathematical
> >>relations theory says nothing about predicates.
>
> > Nope no difference I'm afraid. A proposition that satisfies a
> > predicate P(a,b,...z) is encoded as a tuple of values. All the tuples
> > (rows) satisfying P, are collected as a set, which we call a relation
> > (table). This is sort of database 101 really.
>
> To be fair, Codd didn't establish the equivalence of expression of the
> algebra and the calculus until his 1972 paper.
>
>
>
> >>I can accept that a
> >>tuple of a mathematical relation or a vector can be /interpreted/ as
> >>an existence predicate, but that still leaves the question (see below)
> >>of how a foreign key is (or is part of) a fact or proposition or
> >>predicate.
>
> >>>>So as long as there is no universal consensus about how
> >>>>relational databases and the relational model allow us to represent
>
> >>>But there /is/ universal consensus from people who have read the RM
> >>>paper.
>
> >>If you are speaking of Codd's 1970 paper, I've read it many times and
> >>I /don't/ share your beliefs. In logic, "universal" has a precise
> >>meaning. Unless you have some reason to exclude me from the set of
> >>"people who have read the RM paper", your statement is just rhetoric.
> >>(You could say "most" or "the majority", but you didn't.)
>
> > Then you honestly need to reread the paper Rob. Or get "Introduction
> > to Databases" by Codd, which is more thorough and more up to data. Any
> > database theory primer should do really.
>
> >>>I wonder if you are perhaps you trying to implement a different data
> >>>model inside of the RM mechanism (as people do with EAV)? The PKFK and
> >>>JT "structures" you describe correspond directly to propositions that
> >>>are stated in the real world, but I am at a loss as to what facts your
> >>>more complex A-L "structure" correspond to.
>
> >>I don't know what you mean when you say that 'The PKFK and JT
> >>"structures" you describe correspond directly to propositions that are
> >>stated in the real world'. I'm going to take a stab at it (next), but
> >>I would appreciate some enlightment from you on what fact or
> >>proposition the one foreign key in a PKFK representation child tuple
> >>stands for and what fact(s) or proposition(s) the two foreign keys in
> >>a JT representation stand for.
>
> >>IF I ASSUME THAT:
>
> >>a.) a value X in the foreign key in a PKFK child tuple is the fact:
> >>"this child tuple is related to [or included in the set of child
> >>tuples related to] the parent tuple whose primary key value is X";
> >>b.) a value pair (Y,Z) in the foreign keys in a JT tuple is the fact:
> >>"the parent tuple with primary key Y is related to the child tuple
> >>with primary key Z".
>
> >>THEN FOR THE A-L REPRESENTATION:
>
> >>c.) a value U in the parent foreign key of an aggregate tuple (in the
> >>Aggregate-Link representation) is the fact: "there exists an
> >>aggregation for which the parent tuple with primary key X has a
> >>distinguished role", and
> >>d.) a value pair (V,W) in the aggregate- and child foreign keys
> >>(resp.) of a link tuple (in the Aggregate-Link representation,
> >>associated only with the aggregate relation containing the aggregate
> >>tuple in c) is the fact: "the child tuple with primary key W belongs
> >>to the aggregation specified by the aggregate tuple with primary key
> >>V".
>
> >>This is my best guess as the answer to your (implied) question 'what
> >>facts [do] your more complex A-L "structure" correspond to?'. If I
> >>have misinterpreted the meaning of the foreign keys that you say
> >>'correspond directly to propositions that are stated in the real
> >>world', I apologize. Explain what you mean and I'll try (yet) again.
>
> > You're way overcomplicating Rob - you sound like an OO programmer who
> > came to databases later? Am I right? You have to forget children and
> > parents mate, they are OO concepts and make no sense in predicate
> > logic where there is just inference (It was a wrench for me when I had
> > to make that leap). Anyhow, here is an example for you:
>
> > 1) UK is in Europe => (country:UK, continent:Europe)
> > 2) Canada is in N. America => (country:Canada, continent:N.America)
> > 3) Bob is 30 and lives in the UK => (name:Bob, age:30, country:UK)
> > 4) Sarah is 28 and lives in Canada => (name:Bob, age:30, country:UK)
>
> That's an odd way to represent that proposition.

Agh, the joys of cut and paste, a great tool unless one is both lazy / and/ forgetful.

>
> > 5) Bob is Married to Sarah => (husband:Bob, bride:Sarah)
>
> Since Sarah lives in Canada, the concepts of husband and bride might
> need some adjustment. What happens when there are two husbands? Or two
> brides?

I know nothing about Canada apart from there are a lot of moose, who all play ice hockey and say 'aboot'. I'm visiting in April so I will find out more then ;)

>
> > According to your definitions (1) and (2) are normal propositions (3)
> > and (4) are PKFK and (5) is a JT. Your AL structure corresponds to no
> > statement of fact that I can think of. Regards, J.
>
> And according to RM, of course, they are all just propositions.
>
> >>Rob
Received on Tue Jan 08 2008 - 03:40:16 CET