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Date: Fri, 8 Nov 2002 21:13:43 -0800 (PST)
From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: Ludwig Lim <lud_nowhere_man@yahoo.com>
Cc: PostgreSQL Mailing List <pgsql-sql@postgresql.org>
Subject: Re: Some more weird NULL behavior
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On Fri, 8 Nov 2002, Ludwig Lim wrote:

> Hi:
>
>   I tried the following:
>   CREATE TABLE x(
>        a  NUMERIC(5,0),
>        b  VARCHAR(5)
>   );
>
>   CREATE TABLE y(
>        a  INTEGER,
>        b  VARCHAR(5)
>   );
>
>   INSERT INTO x(b) VALUES ('LUDZ');
>   INSERT INTO y(b) VALUES ('TEST');
>
>   SELECT x.b,y.b FROM x,y WHERE x.a=y.a
>   returns zero rows.

A comparison of two NULL values does not
return true, it returns unknown.
If you want to say equal or both null you
should do it explitly.
(x.a=y.a) or (x.a is null and y.a is null)