Re: A real world example

"Keith H Duggar" <duggar_at_alum.mit.edu> wrote in message news:1155318235.130778.245060_at_m79g2000cwm.googlegroups.com...
> 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.
>
> (Brian note that I'm just a novice. So any questions or
> points I raise are not to argue a position but rather to
> provoke further detail or explanation for my Level 6
> Fraudulent Education.)
>
> Later you say that it illustrates not "the" but rather "a
> similar" problem. Can you provide a real-world example of
> "the" actual problem as Marshall asked?
>
> [snip example of a "similar" but different problem]
>
>> 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.
>
> So isn't the actual problem that the databases employ
> different models of the same data? In other words, it seems
> in that example (and in "the" problem), that the problem is
> not different /values/ but different /models/?
>

No. It's not the data that's the same: it's that the things that the data is about are the same. A database contains information about things in the universe. In a relational database, this information is organized into relations, but that doesn't change the fact that each proposition describes things in the universe. The problem is that a thing can have multiple sets of identifying properties, that some of these identifying properties can have different values in different situations, and that when propositions about a thing refer to it by using identifying properties that can be different in different situations, there's not enough information available to know that you're talking about the same thing.

>> 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.
>
> Why "should [they] correspond"? Are you envisioning some
> meta-data external to the data model that remains constant?
> Some property not modeled that corresponds? If this external
> information and "correspondence" is important, then why is
> it not internalized explicitly as part of the data model?
>
> Just as in the previous example, it seems that in your mind
> you maintain a /different/ model than the database. So again
> the problem seems not to be the changing values but rather
> that the two DBMS (in this case the machine and your mind)
> have /different models/. No?
>
No. See below.
> Can you give a real-world example of two entities sharing the
> /same/ data model having such a correspondence mismatch?
>
> -- Keith -- Fraud 6
>

The universe of discourse is the set of all things that can be discussed. Each element has a set of characteristics, or properties, that define and describe it. As I see it, these properties fall into two categories, individual and universal. An individual property defines the essence of something, either by itself or when combined with other individual properties. Individual properties remain constant because their values depend only on the existence of the thing that they define. For example, someone's DNA sequence could be considered an individual property (though in the case where there can exist identical twins or clones, it may not be sufficient by itself to identify a particular person). A universal property, on the other hand, is descriptive in nature and its value depends on a state of the universe (a situation). The value of a universal property of something can be different in different situations. For example, person's location can be different at different points in time. (I grant that the distinction between individual properties and universal properties assumes that something can come into existence only once.)

So, individual properties define something, and universal properties describe something; it is possible, however, for a set of universal properties to distinguish something from all other things within the scope of a particular situation. For example, consider a queue of people:

(Bob, Brian, Lynn, Susan, Mark, Lisa)

Lynn is third in the queue, so if you say something about Lynn or if you say something about the third person in the queue, then you're talking about the same person. But, if Bob is removed from the queue, you get:

(Brian, Lynn, Susan, Mark, Lisa)

Anything that was said about the third person in the queue now applies to Susan, not Lynn. This is because a person's position in the queue is a universal property, rather than an individual property, even though that property can identify a person within a particular situation. To clarify, consider the following relation schema, QUEUE, with two values, the first representing what was known prior to Bob's removal, and the second representing what is known after:

QUEUE: (DNA sequence, Name, Position)

QUEUE Before:

{(1234..., "Bob", 1),
  (2345..., "Brian", 2),
  (3456..., "Lynn", 3),
  (4567..., "Susan", 4),
  (5678..., "Mark", 5),
  (6789..., "Lisa", 6)}

QUEUE After:

{(2345..., "Brian", 1),
  (3456..., "Lynn", 2),
  (4567..., "Susan", 3),
  (5678..., "Mark", 4),
  (6789..., "Lisa", 5)}

[For the sake of this discussion, please assume that "1234...," "2345....," etc. are a representation of different people's DNA sequences. Also assume that identical twins and clones cannot exist in this universe. While I understand that such a representation is contrived, I'm using DNA sequence as an attribute to prevent the idiotic argument that a person's name can't be a candidate key from detracting from the points I'm trying to make.]

This relation schema has two candidate keys, "DNA sequence" and "Position," because each meets the criteria for a candidate key: its values determine all other attribute values within the same relation value, it is minimal, and these conditions apply to every legal relation value; but it should be obvious that "DNA sequence" refers to an individual property, whereas "Position" refers to a universal property. Confusion arises because the definition of a candidate key ensures only that *statements* about things are unique within any *single* relation value, not that the things under discussion are unique, nor that the values of all of the identifying properties of the things under discussion will remain the same in successive database states. It is possible, therefore, to define a relation schema with a single candidate key that refers to a universal property as one of its domains, for example, Social Security Number.

Since a relation schema can be so defined and since the value of a universal property can be different in different situations, it is possible for propositions in successive database states that describe the same aspect of the same thing to have different candidate key values. In other words, you can't tell if you're talking about the same thing.

The points I was trying to make in the earlier posts was that this inability to correlate the propositions in one database state to those in the next poses several problems. For example, it precludes enforcement of transition constraints. If you can't tell that you're talking about the same thing, then how can you determine whether a new attribute value is legal? I refer you back to those earlier posts for enumeration and examples of the other problems. Received on Sat Aug 12 2006 - 04:32:50 CEST