WHERE clause?

I'm trying to create a log table to track changes to a table.. so I've
created a set of rules that look something like this:

CREATE RULE "m_log_change" AS ON UPDATE TO "machine"
do (
INSERT INTO machine_log (who, date, machnum, col, newval)
SELECT getpgusername(), 'now'::text, old.machnum,
'host', new.host
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);
... more similar INSERT statements with different WHERE clauses..
);

My question is:

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.

Seems like a lot of extra work...

Jim Rowan DCSI DCE/DFS/Sysadmin Consulting
jmr(at)computing(dot)com (512) 374-1143