Re: WWW/Internet 2009: 2nd CFP until 21 September
Date: Fri, 14 Aug 2009 15:44:16 GMT
Mr. Scott wrote:
> The closed world assumption doesn't mean that unknown values are false.
> What it means is that when there isn't a row in a table, the fact
> represented is false. A row with a null is still a row, so the fact
> represented is true.
The CWA doesn't depend on 'rows', only whether a fact is recorded (true) or not recorded (false). The fact that an order has a particular address is a different fact, with a different predicate, from the fact that an order's address is unknown. Trying to express that in an SQL row (maybe I should say table) means the row must have more than one predicate. It is unclear to me when SQL uses one of those predicates and when it uses the other.
(Apparently the SQL standard addressed this at one point, maybe still does, by mentioning in some places that nulls are not values and in others that they are values, link below.):
http://www.dbdebunk.com/page/page/2928212.htm Received on Fri Aug 14 2009 - 10:44:16 CDT