Conditional NOT NULL constraint

Hi all!
Is there a simple way to add a "NOT NULL constraint" to a column without using
a trigger if another column is not null?
Something like this:

CREATE TABLE activity(
id SERIAL primary key,
name varchar not null,
created timestamp not null default now(),
modified timestamp,
created_by integer not null,
modified_by integer
);

alter table activity alter column modified_by set not null where modified is
not null;

I want a constraint which says: "modified_by not null if modified is not
null".

I know I could use a trigger for this, but I'm curious if there is a way to do
this with an index.

--
Andreas Joseph Krogh <andreak(at)officenet(dot)no>
Senior Software Developer / Manager
gpg public_key: http://dev.officenet.no/~andreak/public_key.asc
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