Re: [SQL] WHERE clause?

[Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>]

Jim Rowan <jmr(at)computing(dot)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