Re: Something new for the New Year (2008).
Date: Mon, 07 Jan 2008 21:53:01 -0400
Message-ID: <4782d77e$0$19866$9a566e8b_at_news.aliant.net>
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.
>>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. > >>>>are stated in the real world, but I am at a loss as to what facts your
>>>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
>>>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.
> 5) Bob is Married to Sarah => (husband:Bob, bride:Sarah)
> 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 - 02:53:01 CET