A real world example
Date: Wed, 09 Aug 2006 14:35:18 GMT
Message-ID: <GomCg.4538$o27.3127_at_newssvr21.news.prodigy.com>
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.
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, 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. 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. 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.) Received on Wed Aug 09 2006 - 16:35:18 CEST