- From: Pat Hayes <phayes@ihmc.us>
- Date: Wed, 22 Dec 2004 14:53:47 -0800
- To: Steve Harris <S.W.Harris@ecs.soton.ac.uk>
- Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
>On Wed, Dec 22, 2004 at 05:26:01PM +0000, Andy Seaborne wrote: >> >Yes, but SQL for eg. has tri-value logic (true, false and NULL), so you >> >can meaningfully apply operators and functions to unbound values (NULL). >> >> It doesn't quite work out that simply. It's fine for operators and >> functions but pattern matching isn't so straight forward. >> >> OPTIONAL (<x> ?p ?o) >> (?o ?q <y>) >> >> so ?o may be NULL then we have the (?o ?q <y>) and it needs to handle >> ?o = NULL differently. NULL is different. > >Yes, bun in RDF you cant have a triple like (NULL ?q <y>), so that match >will always fail. Unless I'm missing something. Er...seems to me that you are missing a use/mention confusion. NULL here is supposed to be a value (right?), not a piece of syntax. So what does it even mean to put NULL into a triple? Or are you suggesting that 'NULL' - that is an actual piece of syntax, a four-letter character string - is used as a kind of dummy binding to stand for the binding in cases where a variable doesn't have a binding? If so, I think this really is a very bad idea. All sorts of problems will arise right away: eg two different unbound variables will be the same. It would be much better to simply leave the variable in the pattern in cases like this. > > Talking about NULLs, with all it special cases for matching and function >> handling, like NULL != NULL, is no different to talking about unbound >> variables. Both need special handling. > >There are no special cases. Any arithemtic operation involving NULL is >NULL, so NULL == NULL is NULL, NULL > 3 is NULL, ... Does NULL match NULL? If ?x and ?y are both bound to NULL, are the bindings the same? And so on. Pat -- --------------------------------------------------------------------- IHMC (850)434 8903 or (650)494 3973 home 40 South Alcaniz St. (850)202 4416 office Pensacola (850)202 4440 fax FL 32502 (850)291 0667 cell phayes@ihmc.us http://www.ihmc.us/users/phayes
Received on Wednesday, 22 December 2004 22:55:58 UTC