Re: a union is always a join!

From: Tegiri Nenashi <TegiriNenashi_at_gmail.com>
Date: Wed, 18 Mar 2009 22:00:32 -0700 (PDT)
Message-ID: <557d6f32-dccf-4512-b74e-2e6e1cb064a0_at_w35g2000prg.googlegroups.com>


On Mar 18, 6:25 pm, br..._at_selzer-software.com wrote:
> I think that the delete/insert pair states that
> there is a different thing with key p = 1, whereas the update states
> that the thing with key p = 1 merely appears different.  In other
> words, the delete/insert pair describes two distinct things, whereas
> the update describes two distinct appearances of the same thing.  The
> delete/insert pair,
>
> delete {(p=1,q=a)}
> insert {(p=1,q=a)}
>
> states that there are two distinct things with exactly the same
> components, just during adjacent intervals.

Why do you need to identify "things"? Unless you made a typo, please also note, that I'm not allowing arbitrary deletions and insertions. Insertions and deletions in my understanding are just differences between old and new state, and vise versa. Therefore, deleting and inserting the same relation {(p=1,q=a)} doesn't make any sense.

> Unfortunately, it just isn't that simple.  Since the key components
> that identify something can be different at different times yet still
> identify that same something, there can be more than one transition
> that yields a resulting state.  For example, if the guy that had up to
> this point been first in line at the bank was wearing a blue hat, and
> if the guy that is now first in line is wearing a red hat, then one
> possibility is that there is a different guy that is first in line,
> the guy wearing the red hat, but another possibility is that the guy
> that had up to this point been first in line is still first in line
> but just put on a red hat.  So which is it?  If the transition
> consisted of a delete/insert pair, then it's clear that it's the first
> possibility, but if the transition consists of an update, then it's
> clearly the second.

This "real life" example is not helping. I doubt customer hat is of any concern to the bank business (unless it is ski hood), much less to the general public. Received on Thu Mar 19 2009 - 06:00:32 CET

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