[std-interval] Comparisons

[George Corliss]

As this groups does not yet appear to have reached a consensus on comparison
operators, there are some additional thoughts.

I've been asked to teach a database class for the first time in my career,
so I am reading about SQL (Structured Query Language).  Interestingly, SQL
uses a three-valued logic with values TRUE, FALSE, and UNKNOWN.

The rationale seems to be
   1. Fields in a database often have values NULL
   2. Comparisons with NULL yield UNKNOWN
   3. Need "Boolean" operators on results of comparisons

The analogy with intervals is not perfect, but it is informative.  Since
databases are more widely used than intervals, their standards have endured
severe scrutiny and extensive practical applications.  We may have something
to learn.

Dr. George Corliss
Electrical and Computer Engineering
Marquette University
P.O. Box 1881
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Milwaukee, WI 53201-1881
George.Corliss at Marquette.edu
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Www.eng.mu.edu/corlissg