Re: Does Codd's view of a relational database differ from that ofDate& Darwin? [M.Gittens]

Gene Wirchenko schrieb:
> On Tue, 07 Jun 2005 17:42:23 +0200, Alexandr Savinov
> <savinov_at_host.com> wrote:
>
> [snip]
>
>

>>To say "if NULL means unknown" is the same as to say "if 0 is equal 1" 
>>or "if empty set is full set". Unknown and null are two different things 
>>and deserve to have to special designations.
>>
>>NULL means absence
>>UNKNOWN means presence with unknown identity

If I understand you correctly then this should be designated as UNKNOWN. If myVariable=UNKNOWN then some *single* value is known to be assigned to this variable but it is not specified and cannot be found out.

If we assume that the variable is set-valued then UNKNOWN has another interpretation. If childrenOfJohn=UNKNOWN then this means that John is known to have some set of children but this set is not specified and cannot be found (at the moment). One option here is if we allow for childrenOfJohn to have empty set of children if it is unknown.

-- 
alex
http://conceptoriented.com