Re: temporary databases

"Bob Badour" <bbadour_at_golden.net> wrote in message news:<4fXL8.25117$H%2.196428937_at_radon.golden.net>...

Welcome back, Bob.

> For all practical purposes, all intervals within a DBMS represent a set of
> discrete points. As a result, a tuple that has an interval attribute
> represents a set of discrete propositions. In the absence of any interval
> attributes, a tuple represents a single proposition.

Interval represented as a set of discrete points is just one possibility, and probably not the best one. Given an interval defined by its 2 boundary points and discretisation step, one can always query all discrete points within the interval.

Therefore, I would represent an interval by just 2 timestamp columns. It is unnecessary to consider it as a single object with 13(!) operations.  

> This affects the integrity function, the manipulation function etc.
>
> Consider an HOURS relation with a single interval column representing the
> hours of operation and an APPT relation with an interval column representing
> the time and duration of appointments plus other columns identifying the
> customer and reason etc.
>
> One would like to declare an integrity constraint that the discrete points
> represented by any APPT interval are a subset of the points represented by
> the HOURS relation. Likewise, one would like to declare an integrity
> constraint that the discrete points represented by all APPT intervals are
> unique within APPT. One would like uniqueness within the discrete points of
> time represented by the HOURS relation too.

More simple than that:
1. Start and end of an appointment interval both are within HOURS. 2. Neither start nor end point of an appointment falls within some other appointment.

One motivation to consider an interval as a set of discrete points might be the fact that sets and subsets are a sweetheart of the relational model. IMHO the discrete topology of time line is a wrong model.  

> These constraints are very similar to a foreign key constraint and a
> candidate key constraint, but they are subtly different due to the
> intervals. For instance, a traditional foreign key constraint would require
> that the HOURS relation have the exact intervals in the APPT relation. A
> traditional candidate key constraint would not prevent overlapping intervals
> and would thus not enfore uniqueness among the discrete points represented
> by the intervals.

Yes, traditional foreign key constraints emphasize equality, and we would like to focus on the constraints with inequalities, instead. However, I fail to see any temporary specifics in that.  

> I recently attended a fascinating seminar by Chris Date on the subject. He
> is holding the same seminar the week of June 10th in Rome if you are in
> Europe and can afford to go. (http://www.technologytransfer.it/en/seminars/)
>
> He mentioned that he is writing a book on the subject that should hit store
> shelves Q4.

Is any of his recearch published (in a series of articles)? Given that Snodgrass book is collecting dust somewhere on my bookshelf, I would be very careful this time. Even if the author is Chris Date. Received on Fri Jun 07 2002 - 19:34:47 CEST