Re: NULLs: theoretical problems?
Date: Sat, 18 Aug 2007 20:12:50 +0100
"David Cressey" <cressey73_at_verizon.net> wrote in message
> "David Portas" <REMOVE_BEFORE_REPLYING_dportas_at_acm.org> wrote in message
>> "David Cressey" <cressey73_at_verizon.net> wrote in message
>> SELECT DISTINCT i FROM t WHERE x = x;
>> The result is clearly wrong if a null means "x is unknown" or "x is out
>> range" and possibly wrong if null means "x does not apply".
> But if a null means "x is not present" and nothing more, the result is
> clearly right.
Giving yet another interpretation of null demonstrates only that there is no generally accepted interpretation. What on earth is "not present" supposed to mean in a proposition anyway? I suggest that it is far from clear that "not present" means that x = x evaluates to something other than true!
>> If you admit nulls then you must deal with predicates that reference
>> nullable attributes.
You are seriously suggesting that nulls should never appear in predicates? That alone must be a strong argument to banish them entirely.
-- David PortasReceived on Sat Aug 18 2007 - 21:12:50 CEST