Re: WWW/Internet 2009: 2nd CFP until 21 September x

Walter Mitty wrote:
...
> Where a lot of database designers go awry is to assign "meaning" to missing
> data. For example, if spouse's first name is missing, it means the person
> has no spouse. People who set up information systems with this kind of
> conventional interpretation are entering the quicksand. I guess, to come
> back to your comment of a few replies back, this means that I share your
> qualms about the CWA. If there is no passenger named Yussuf Islam on the
> passenger list, it could mean that there is no such passenger on the plane,
> or it could mean that the passenger list is incomplete or incorrect. (I may
> be misrepresenting your position on the CWA, but you get the idea.)

You are misrepresenting it. I have more than qualms about the OWA, not the CWA.

1. db meaning is expressed by predicates (and to be precise, meaning isn't assigned to data, rather data (values) are substituted in the predicate's place-holders, giving a statement that by the CWA convention is true. If the predicate is "spouse's first name is X" and there is no value to substitute for X, the meaning is that there are no spouses's first names. That predicate doesn't mention anything at all about there being no spouses, the sentence "there are no spouses" is not even a predicate, it is a proposition. This is very simple, simple enough for a child to learn but adults like Scott are so easily tripped up, like trying to substitute four values into six placeholders don't ask me why, maybe it's because they confuse a big vocabulary with basic understanding.
2. it might be quite reasonable in some particular circumstance to conclude that somebody isn't on the plane because his name isn't on the passenger list, but if the db designer has told the users that the predicate is "X is on the passenger list", that is the only predicate expressed by the db. Concluding X is not on the plane has nothing to do with the db. This is the essential agreement required by the CWA. Whereas if there are no values for X, the OWA allows any user to conclude that somebody is on the passenger list, regardless of what's recorded in the db. Actually, people who subscribe to the OWA, don't really need a db, lucky them.
3. Whether db information is incorrect is nothing to do with RT. Whether it's incomplete is very much to do with RT, that's why we have constraints. Don't remember if I mentioned it before, but I think whenever talk of incomplete data comes up, the best way to be clear about whether a given constraint handles it is to phrase it in the algebra and then ask whether the predicate's place-holders are satisfied.