Re: Who invitented the division operator in relational algebra and why?
From: --CELKO-- <71062.1056_at_compuserve.com>
Date: 17 Apr 2003 15:10:08 -0700
Message-ID: <c0d87ec0.0304171410.1f89c7e7_at_posting.google.com>
Date: 17 Apr 2003 15:10:08 -0700
Message-ID: <c0d87ec0.0304171410.1f89c7e7_at_posting.google.com>
>> Does anyone have references to the first article where the division
operator
was "made"? <<
Dr. Codd made it one of his original eight operators ("which suppliers sell this list of products?"). Then Todd defined another relational division, based on subsets that match ("which suppliers sell the same products as each other?"). The best one is due to Richard Romley; it returns a flag to tell you if the sets are exact matches, if one is superset of the other or if they are disjoint ("all", "some", and "none").
I have a chapter in SQL FOR SMARTIES that goes into several ways to do various relational divisions. Received on Fri Apr 18 2003 - 00:10:08 CEST