Re: So what's null then if it's not nothing?

<s.j.lagoe_at_googlemail.com> wrote in message news:1134267146.767667.96760_at_g43g2000cwa.googlegroups.com...
> Hello, I have 1 question about the 3VL that hopefully someone can
> answer. With data that we cannot avoid is missing we may have a
> true/false column that can have nulls in it which gives us a 3VL so
> that:
>
> ((column_1=T) && (column_2=T)) = T
> ((column_1=T) && (column_2=F)) = F
> ((column_1=F) && (column_2=F)) = F
> ((column_1=T) && (column_2=?)) = ?
> ((column_1=F) && (column_2=?)) = ?
> ((column_1=?) && (column_2=?)) = ?
>
> Now, I read in the thread that for this to be a sensible logic we have
> ?=?. Ok, but that means that an unknown equals another unknown. That
> does not seem to make sense because by saying ? we at the start said
> that we dont know values... so how can we compare them at all? thank
> you, Sigmund.

Let's consider different representations of numbers 1={one, I, un} 2={two, II, deux} 3={three, III, trois} etc.
Then consider this test: X = Y ?
one = I ?
one = un ?
I = un ?
one = two ?
one = deux ?
Then consider NULL = 1 union 2 union 3 union .... If X is in 1 and Y is in 1 then X=Y is in True. If X is in 1 and Y is in 2 then X=Y is in False. If X is in 1 and Y is in NULL then X=Y is in Unknown. If X is in NULL and Y is in NULL then X=Y is in Unknown. Received on Mon Dec 12 2005 - 15:15:44 CET