Re: A pk is *both* a physical and a logical object.

On Jul 12, 8:21 pm, "David Cressey" <cresse...@xxxxxxxxxxx> wrote:
"Cimode" <cim...@xxxxxxxxxxx> wrote in message

news:1184260798.451810.60010@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



On 12 juil, 18:25, "David Cressey" <cresse...@xxxxxxxxxxx> wrote:
"Cimode" <cim...@xxxxxxxxxxx> wrote in message

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On Jul 12, 2:15 pm, "David Cressey" <cresse...@xxxxxxxxxxx> wrote:
"Jan Hidders" <hidd...@xxxxxxxxx> wrote in message

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On 11 jul, 22:25, Cimode <cim...@xxxxxxxxxxx> wrote:
Furthermore...
<<Technically a PK is *only* a physical implementation device,
not a
logical concept at all.>>

`When I use a word,' Humpty Dumpty said, in rather a scornful
tone,
`it means just what I choose it to mean -- neither more nor less.'

`The question is,' said Alice, `whether you can make words mean so
many different things.'

`The question is,' said Humpty Dumpty, `which is to be master --
that's all.'

;-)

To answer the question, I think that is quite simple. As defined
in
the relational model it is a logical concept. As far as I know the
SQL
standard does not state that a PK implies an index (but I could be
wrong) and then it is also there a logical concept. If it does
imply
an index then it is mixed concept because it has both logical and
physical consequences.

-- Jan hidders

It was my understanding that the relational model defines keys, but
not
primary keys. That is, any candidate key is as much of a key as any
other.
In a relational perspective, the term *primary key* was first used by
Codd to designate a specific unique identifier that allows to
distinguish tuples.

It was my understanding that certain schools of data management,
including
the SQL school, adopted the convention of naming one candidate key
as
primary key, and of making all FK references refer to that key,
where
possible. I can see, and use, that practice myself. But I can't
see
where
the relational model necessitates it.
Who cares what SQL schools of *management* have to say about
relational model?

Please don't confuse "management" with "data managment".
I think I misread your comment (did not mean to sound offending).

Yes, I think you did.
Sorry about that but a fact is a fact. *SQL school* does not have
anything productive to say about theory of relations.

I
thought you were advocating that SQL schools of data management have
any relevance anymore as to what should and should not be relationally
sound. Quite frankly, I do not pay any attention to anything that is
SQL related (except to make a living)..

SQL oriented people (I might include myself) will advocate declaring a
primary key whenever a table is created, and making all FK references to
the table via its primary key. That turns out to be of value even if the
primary key concept is outside of the relational model of data.
I do not think the concept of primary key is anywhere *outside* the
relational domain of investigation of database management as its
concept of unique identifier was defined in relational theory well
before SQL systems were born.

I don't see the relevance nor intellectual honnesty into appropriation
by SQL oriented people of the concept of unique identifier. For
instance, tha fact that I use SQL on a daily basis because I need to
make a living, does not prevent me from seeing how SQL breaks
relational rules at every level and how SQL people (ANSI committee)
perverted relational model probably because the were pressured by
dbms editors. Considering the recent trends in dbms products, I would
not be surprised for instance that XML would become a new SQL standard
proponent. That would be a total antithesis to relational theory and
a major regression in database management in general.



.

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