Re: ID field as logical address
Just a couple of comments about a post that is unusually vague and fuzzy
coming from you, it's tedious to dismantle every sentence, so I mention
only a couple:
Walter Mitty wrote:
> ... Now let's say
> you have a table that records values that look like (ID, value). In an
> actual case, value would probably ityself be a tuple, ...
Sounds like you are talking about a tuple within a tuple, the tuple that
'value' stands for being somehow 'within' the tuple that '(ID, value)'
stands for. I don't think there is a relational notion that matches
this, maybe you actually have RVA's in mind.
...
> So, Brian's claim that deleting a value and inserting a new one is somehow
> really different from updating an existing value works in this model, if
> you regard ID as somehow "special", and not "part of the value recorded in
> the table".
> ...
He is not the only one who talks about one thing and means something
diifferent. Here, at least one of you is dreaming of a table that
doesn't stand for one of Codd's relations. (Apparently SQL is capable
of the same thing, for reasons such as unnamed columns. According to
Date another flaw was that SQL is based on 'bags', not sets so it
wouldn't be a surprise if Codd shunned the System R department when he
worked for IBM.)
The first thing the mystics forget is that they can't make their claims
without admitting that they are talking about something other than
conventional relational theory. Their claims should be dismissed until
they develop their own formal theory for whatever machinations they have
in mind and make their assumptions and variations precise. Relational
theory is still developing but people who refuse to give substitutes for
the very few formal concepts only get in the way of progress.
Meanwhile, aspects of Codd's theory have loopholes such as whether some
views can be 'updated', constraint theory is mostly undeveloped, etc.
The majority of posts to c.d.t. are about physical questions. Regarding
the topic at hand, I've never seen it asked here whether there is any
logical reason why views can't have generated attribute values If a
view can have D&D-style extend-ed attributes, is there a use for
generating values? One possible use might be to simplify quota queries.
(Depending on how you read Codd's 1970 paper, he might have left the
door slightly ajar when he described his reduction algorithm, because
one can imagine hierarchies where arbitrary attributes need to be
introduced. If there are hierarchies that can't be shown as relations
without introduced attributes, are there relations that can't be shown
as tables without introduced attributes? I think Codd might have said
that if there are such relations, they should be avoided, but that's
just a guess. I'm certain he would have said such situations weren't
the problem he was after solving. Nevertheless, Codd never precluded
such attributes as fas as I know, eg. he didn't forbid an attribute's
value being generated by the machine.)
Received on Fri May 29 2009 - 18:39:55 CEST
Original text of this message