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To: Jim Rowan <jmr@computing.com>
cc: pgsql-sql@hub.org
Subject: Re: [SQL] WHERE clause? 
In-reply-to: Your message of Wed, 14 Jul 1999 00:17:12 -0500
	<199907140517.AAA23407@zee.computing.com>
Date: Wed, 14 Jul 1999 10:00:02 -0400
Message-ID: <25399.931960802@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Jim Rowan <jmr@computing.com> writes:
>     WHERE (new.host != old.host) or 
> 	(old.host IS NOT NULL and new.host IS NULL) or
> 		(old.host IS NULL and new.host IS NOT NULL);

> Is this the best way to code the WHERE?  What I'm really after is "did the
> value change?".  I found that the tests to see if one-but-not-both of the
> values is NULL are required to identify times when the value changed to or
> from NULL.  

Yes, because any ordinary operator applied to NULL will produce NULL,
which WHERE interprets as FALSE.  The only operations that actually
work on nulls are IS NULL/IS NOT NULL.  In 6.5 you can use COALESCE
as a handy abbreviation for certain sorts of IS NOT NULL tests, but
offhand I don't see a good way to apply it here.

			regards, tom lane