On Mon, Jun 04, 2007 at 12:37:42AM +0200, PFC wrote: > NULL usually means "unknown" or "not applicable" Aaaargh! No, it doesn't. It means NULL. Nothing else. If it meant unknown or not applicable or anything else, then SELECT * FROM nulltbl a, othernulltbl b WHERE a.nullcol = b.nullcol would return rows where a.nullcol contained NULL and b.nullcol contained NULL. But it doesn't, because !(NULL = NULL). It's too bad indeed that the originators of SQL used three-value rather than five-value logic, but this is what we have. If you happen to want to use NULL to mean something specific in some context, go ahead, but you shouldn't generalise that to "usually means" anything. A -- Andrew Sullivan | ajs@xxxxxxxxxxxxxxx Everything that happens in the world happens at some place. --Jane Jacobs
