column: on update update?

Hi, I fear this is a beginners question but I'm no SQL guru and couldn't
find it out of the documentation.

For SpamAssassin, I have a table "awl" (auto white list), and there
should be one field "lastupdate". I added it like this:
ALTER TABLE awl add column lastupdate timestamp default now();

But I also need the "lastupdate" be updated every time a field in that
row is updated, so a statement like this:

ALTER TABLE awl add column lastupdate2 timestamp default now() on update
set now();

But it doesn't work. Do I need a trigger? If yes, what would be the
format? I never did triggers.

mfg zmi
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