Trigger help - updates to column data with null values

Hello all!
I do my best to read and google my way around issues, but I seem to be
missing something. Probably simple too.

So I have a trigger function, in which I'd like to check if a particular
column has changed. It works great, unless either the OLD or NEW values are
NULL. The purpose is to create a audit/log of the record. I prefer to only
test for certain columns, not the whole table.

CREATE OR REPLACE FUNCTION tr_employees_bu()
RETURNS trigger AS
$BODY$
begin
if (new.trainingdate <> old.trainingdate) then
insert into log_employees (employeeid,
source, datecreated,
createdby, oldvalue,newvalue)
values
(old.employeeid,'trainingdate',
now(),new.updatedby,
cast(old.codetrainingdate as varchar),
cast(new.codetrainingdate as varchar));
end if;
end;
$BODY$
LANGUAGE 'plpgsql' VOLATILE
COST 100;

I tried playing with "coalesce" and "nullif" but didn't get anywhere with
those. I think the "if" expression is not right.

Thanks for any help!

Greg Fischer
www.1stbyte.com