Subject [ib-support] Re: expression evaluation Lauri Zoova 2002-12-22T13:23:44Z
Since you raised a few new questions i'll try to explain myself:

Helen Borrie wrote:
>>>[...]
>>>Just remember that if new.field and old.field are both null, you won't get
>>>True on an equivalence comparison either...
>>
>>So we are back at the beginning :)
>
> We are at fundamentals, I suppose you could say. Unknown == unknown cannot
> be evaluated as equal, e.g. Lauri's phone number == Helen's phone number if
> you don't know either of them.
> [...]
> How do you consider "valueless state" and "unknown" to be different, logically?

Extending the phone number example:
Lauri's phone number = Helen's phone number, if i know that neither of
us has a phone number. This is the case with null. (IMHO)

>>I really can not think of any use for unknown state or a boolean
>>condition that can evaluate to unknown. I guess it's my limited mind :))
>
> Turn your thinking around the other way. The state of data is always
> KNOWN: it has a value or it has no known value. It would be very
> restrictive if "no known value" were predicated as if it could be evaluated
> as "something" - apart from the untruth of making null pose as some sort of
> value.

No. There should not be "no known value", but "no value" and "no value"
should be considered different from "value" not unknown. The possibility
of unknown result in boolean evaluation is what i'm against of.
There can't be a field has a state of null and at the same time has a
value, correct?

And (un)truth is only ones opinion.. so it has no real value - truth is
null :P

BR,
Lauri