Re: A real world example

Brian Selzer wrote:
> Facts are facts about things. If things can change, then the facts about
> them must also or must be replaced.

I think the "thing-orientation" here gets in the way of thinking about facts as instances of predicates that we care about. When we talk about the database, it's the predicates that are primary, not the things they concern. A change in the database doesn't necessarily reflect a "thing changing", unless you collapse "thing" to mean predicate rather than some entity.

> > Maybe, but from a functional standpoint, that operator is just a
> > function (e.g. "subtract $500 from X), in which the balance is a free
> > variable. Only in an imperative world does that involve "knowing"
> > (referencing) the "previous" balance. Function application means
> > there's no "query" of the value prior to the update.
>
> Not necessarily. For example, consider a sales order that can have several
> states, proposed, open, firm, shipped, received, closed, cancelled. Assume
> that the order stated is the normal set of state changes for the order. Now
> consider that an order that cannot become proposed once it is firm, it
> cannot become received unless it has been shipped. It cannot become closed
> unless it has been received. Unless you define special operators to deal
> with the states, you need to know what the old value was in order to
> maintain the consistency of the database throughout the update.

Maybe the discrepancy hinges on the phrase "you need to know." I'd argue that no query is needed, merely constraints.

> >> More
> >> importantly, the bank must be able to identify the account that is about
> >> to
> >> change, and that identity must remain constant in both the preceding and
> >> succeeding database instances.
> >
> > Why? As long as it can be identified via some query, what difference
> > does it make? For example, if I make a database schema change and
> > introduce a new key, with appropriate view changes to support old
> > application code, is there some logical distinction? If the external
> > queries all still produce the same results, excepting the specific
> > values being updated, what does "identity" have to do with it?
>
> Because changes are set-based, and if the identity of the account can
> change, then it's possible to update the wrong row, or to allow a charge to
> clear that shouldn't be allowed.

There is no "wrong row," only a set of propositions. The same possibility for human error would seem to be present in any update: that you might issue an update without knowing about a change made between the time you last loaded the page, and the time you pressed Save, and therefore could violate a constraint which you wouldn't violate if only the database were in the state you think it is (based on what's on the screen). This issue seems to be a particular variant.

> > There is no "thing." These are propositions, or assertions if you like,
> > nothing more. The only meaning is in the correlation of queries to
> > external phenonema of interest.
>
> What are the propositions or assertions about? If they're about values then
> they're just hot air. A database contains knowledge. Knowledge about what?
> Scalar values? I don't think so.

They're about what is in our heads - the application (business) domain. The database doesn't care about that; it's in crafting predicates and constraints that we tell the database as much as it needs to (or can) "know."

> The relational model doesn't have a correct theoretical mechanism to
> correlate tuples during updates. The scope of a key value's ability to
> identify a tuple is a single relation value from a single database instance.
> I think that the model is incomplete without such a mechanism, because there
> are some constraints that cannot be enforced, and certain update anomalies
> can occur, as I've provided examples of in other posts.

Since we're not talking about a machine that "really knows" the real world, I don't understand what sort of mechanism you have in mind - what is an example of a "correct theoretical mechanism"? The relational model already allows surrogate keys.

> True, but it felt really good to spank Badour after all of the abuse he's
> dished out, unprovoked for the most part, I might add. I really loved the
> fact that he doesn't even understand the definition of a candidate key!

I didn't gather from that exchange that he had, but don't feel like diving into that particular argument...

• erk