Andrew Sullivan wrote:
> 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).
Well, a strict "unknown" is fine - so long as it means just that.
How tall is Andrew? Unknown
How tall is Richard? Unknown
Are Andrew and Richard the same height? Unknown
The problem is the slippery-slope from "unknown" to "not applicable" to
"user refused to answer" to ...whatever
Part of it is the poor support for out-of-band values. In many cases
what people want is the ability to have a value of type 'number in range
1-20 or text "n/a"' and there's not a simple way to provide that, so
they use null.
--
Richard Huxton
Archonet Ltd
