Re: A real world example

Brian Selzer wrote:
In an earlier thread, Marshall asked for a real-world example, and while
this isn't exactly what was asked for, it illustrates the problem. Here's
the situation. I have two databases containing orders. One is the
customer's database containing purchase orders, lines, schedules and
releases. The other is the supplier's database containing sales orders and
lines. There should be a 1:1 relationship between the releases in the
customer's database and the lines in the supplier's database.

If you join the respective tables in each of the databases, you get the
following:

Customer purchase order detail:
{PO#, POLine#, CustPart#, Schedule#, ReqestedDate, RequiredQuantity,
ReceivedQuantity}

The primary key is {PO#, POLine#, Schedule#, RequestedDate}

Supplier sales order detail:
{SO#, PO#, SOLine#, Part#, CustPart#, RequestedDate, PromiseDate,
RequiredQuantity, ShippedQuantity}

The primary key is {SO#, SOLine#},
an alternate key is {PO#, CustPart#, RequestedDate, SO#}

The first problem is that the customer uses a blanket PO, and therefore,
(usually) reuses the same PO# and Line# to order a part--the schedule number
differentiates one set of releases from another.

The second problem is that the supplier's database doesn't track schedules,
so there's no way to differentiate sets of releases, except that the SO#
will be different for different schedules.

Aside from those there have been keying errors, some rows don't have a
customer part #, some rows don't have a PO number, etc.

What I've been asked to do is to produce exception reports, showing what is
different between the two databases so that changes to purchase orders that
appear in the customer's database can be verified after they're entered into
the supplier's database.

It may be tiredness, but I found that example hard to digest. However
if I understand your gist correctly, you are pointing out a situation
where an external entity is identified differently, in two distinct and
separate databases? If so, then this will always necessarily require a
'situated' sentient-being to act as middleware. After all a db doesn't
represent a universal truth about the real world, it just stores an
encoding of it acceptable enough for a target user to interpret what it
tells them in a useful way. In the end its always the human user
putting the semantic pieces back together.


My point is that here is a real-world example where the universes of two
databases overlap, but the set of attributes used to identify something
common to the two universes is different for different databases. I
understand that this is a common problem when merging or otherwise
connecting databases, and I've run into this numerous times and have it
covered, but the fact remains that a similar situation can occur within a
single database. With natural keys, that is, keys whose value can be
different in successive database states, it is possible for the values of
one set of identifying attributes of something in the universe to be
different in successive universe states

huh? different identifying attributes, different things. How can they
be the same, unless there is an incorrect key choice? Liebniz equality,
etc., etc. (I know you've heard this from me before now, so apologies
for the repetition, but I still can't see how I'm not connecting with
you on it). The successive database states example, unless i've
misconstrued it, appears analagous to talking about a company who uses
a primary key of firstname for its employees relation, only for them to
realise a year later that they've cocked up when they hire someone with
a clashing firstname. Thats no mistake in the theory, just in the
coconuts trying to apply it.

, and when this new information is
imparted to the database, it is possible for there to be propositions in
successive database states that should correspond, but cannot because the
only relevant set of identifying attributes of something is different in
successive universe states.

Again I'm foxed by this Brian. How can propositions with different keys
correspond between database states? That notion seems nonsensical to
me. Only entities can 'correspond', and of course they are part of the
conceptual not logical model right?

It doesn't matter how stable a set of
identifying attributes is, if there's the slightest chance that its values
can be different in successive universe states, you have to assume that they
will.

Well, only if there is sufficent risk involved to necessitate worrying
about such a situation for that specific database. It is _possible_
that two people's DNA may match for example, but the chances are so
absurdly infintessimely small that it is of no concern.

The problem is one of relevance. If the DNA of a person is not
relevant, but is the only set of attribute values that is guaranteed to
remain constant throughout the discussion, then what do you do? If you need
to know what was known about a thing, then there are only three choices: (1)
use a surrogate for the DNA and make it the primary key, (2) identify each
statement in the database so that it can be discussed in successive database
states (in other words, use a surrogate for each statement), or (3) define a
tuple-level assignment operator (A tuple-level assignment operator would
permit the system to correlate tuples in successive database states and to
act accordingly. For example, Oracle has a FOR EACH ROW trigger.)

Naturally (1) (with that 'if there is sufficient risk' caveat) - after
all, that's what a surrogate is for. A replacement for an unobtainable
distinguishing attribute. It is the only option which makes any sense
to me logically. However you do not specify which you would choose? My
impression from your previous posts is that you would also choose (1)
but would desire it to be hidden? All best, J.

.