On Wed, 5 Jun 2002, Martijn van Oosterhout wrote:
> One thing I don't know and that is how closely SQL follows relational
> algebra. Is it close enough that you can prove results in relational algebra
> and have them work in SQL. Or there enough special cases to make that
> tricky.
Ha! Nowhere near close enough. SQL actually breaks the relational model
in many ways; C. J. Date's books (particularly _The Third Manifesto_ and
his _Introduction to Database Systems_) go into this in detail.
I've not really had time to sit down and think about the characteristics
of the problem of whether or not a WHERE clause on a pair of SELECT
statements joined by UNION is distributitive or not. The easiest
thing would be if someone could find a case that proves by example
that the WHERE clause is not distributitive.
Providing a formal proof that it is distributive seems to me rather
difficult, because I can't even really see how to translate SQL
into relational algebra....
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC
