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

In article <42a5c072$1_at_news.fhg.de>, savinov_at_host.com says...
> Jon Heggland schrieb:
> > Yes, if NULL means unknown.
>
> 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.

That depends on how you define "NULL". You define it as absence; other people obviously define it otherwise. It is of course a problem. The point in this thread is how SQL defines it. ("Non-empty set" is a better term than "full set", by the way.)

> NULL means absence
> UNKNOWN means presence with unknown identity

Not in SQL. Anyway, you subscribe to Codd's I-marks and A-marks, and 4VL? What about "unknown whether absent or not"?

-- 
Jon