Re: NULLs: theoretical problems?

From: David Cressey <cressey73_at_verizon.net>
Date: Sat, 18 Aug 2007 08:58:57 GMT
Message-ID: <lxyxi.34$wr3.33@trndny04>

"David Portas" <REMOVE_BEFORE_REPLYING_dportas_at_acm.org> wrote in message news:l_Cdnbfs5KQfllvbRVnytgA_at_giganews.com...
> "David Cressey" <cressey73_at_verizon.net> wrote in message
> news:UGmxi.11$dz3.4_at_trndny01...

> SELECT DISTINCT i FROM t WHERE x = x;
>
> The result is clearly wrong if a null means "x is unknown" or "x is out of
> range" and possibly wrong if null means "x does not apply".

But if a null means "x is not present" and nothing more, the result is clearly right.

>
> If you admit nulls then you must deal with predicates that reference
> nullable attributes.

Why? Received on Sat Aug 18 2007 - 03:58:57 CDT