Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!drn.maxwell.syr.edu!news.maxwell.syr.edu!newsfeed.icl.net!newsfeed.fjserv.net!news.tele.dk!news.tele.dk!small.news.tele.dk!uninett.no!ntnu.no!not-for-mail From: Jon Heggland Newsgroups: comp.databases.theory Subject: Re: So what's null then if it's not nothing? Date: Mon, 12 Dec 2005 11:08:33 +0100 Organization: IDI/NTNU Lines: 36 Message-ID: References: <1133896860.912140.197720@f14g2000cwb.googlegroups.com> <1133992308.569062.326920@g14g2000cwa.googlegroups.com> <1134052281.291970.61920@g44g2000cwa.googlegroups.com> NNTP-Posting-Host: coleburn.idi.ntnu.no Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-15" Content-Transfer-Encoding: 7bit X-Trace: orkan.itea.ntnu.no 1134382109 31179 129.241.111.99 (12 Dec 2005 10:08:29 GMT) X-Complaints-To: usenet@itea.ntnu.no NNTP-Posting-Date: Mon, 12 Dec 2005 10:08:29 +0000 (UTC) User-Agent: MicroPlanet-Gravity/2.60.2060 Xref: dp-news.maxwell.syr.edu comp.databases.theory:35064 In article , hugo@pe_NO_rFact.in_SPAM_fo says... > On Fri, 9 Dec 2005 23:54:33 +0100, Jon Heggland wrote: > > But you're correct - in the SQL-99 standard, a BOOLEAN (sic!) datatype > is introduced. According to the description in the standard, a boolean > value is either True or False. "The truth value of _unknown_ is > sometimes represented by the null value". Yep, according to Codd's bad precedent. > There is a reson to object to the use of "the null value" to represent > the truth value Unknown. Elsewhere in the standard, the Null value is > defined as "A special value that is used to indicate the absence of any > data value". Of course, if Unknown is considered to be a data value for > a boolean data type (and the rest of the standard clearly indicates it > is), it should not be represented by the same symbol that is also used > to represent the absence of a data value, since a data value can not be > absent and present at the same time. I agree. Codd does not. Thanks a lot for the summary of SQL booleans, but you didn't mention the most interesting thing: When you check two SQL-boolean attributes for equality (using '=', in a WHERE clause), and both are Unknown, what is the result? True or Unknown? > Removing the row and column for NULL from the truth tables and replacing > them by a line that says that NULLs propagate changes only the > representation of the tables, not the content. No. NULL is not part of the domain; it's part of the meta-language. :) Sophistry, yes, but no worse than the convoluted definitions and rules for how NULLs influence other parts of the system. -- Jon