On Thu, Jun 08, 2006 at 06:09:21PM -0700, Trent Shipley wrote: > On Thursday 2006-06-08 15:14, David Fetter wrote: > > On Thu, Jun 08, 2006 at 05:21:07AM -0700, dananrg@xxxxxxxxx wrote: > > > on bag theory[1] and 3-value logic[2]. Until they come up with a > > testable system, or Hell freezes over, whichever comes first, > > Pascal's book will make a good companion on your shelf to books on > > Phlogiston[3] theory, or a decent doorstop, whichever you prefer. > > I have encountered at least two commercial database products that > declared every column "NOT NULL". I have always assumed that this > was defensive, preventing stupid programmer mistakes. It's not that simple. If there are no NULLs allowed anywhere, that means that you can't even have them as the output of a SELECT statement, which means no OUTER JOINs. No repetitions means none anywhere, which means that they can't be the output of a query either, and makes it complicated at best to do aggregates. The whole thing is just ridiculous on its face. > I recall reading somewhere that Codd proposed multiple flavors of > nullity. Are there theoretical proposals for databases with logical > systems having more than three values? Codd proposed two different NULLs, if I recall right, and some people have come up with tens of different meanings that NULL might have, which leads to some major silliness wherein your "truth table" is a grid that doesn't legibly fit on a piece of A4 paper. Cheers, D -- David Fetter <david@xxxxxxxxxx> http://fetter.org/ phone: +1 415 235 3778 AIM: dfetter666 Skype: davidfetter Remember to vote!
