How can I do this?

I want to have a table (a category table in this instance) that will do the
following:

reference a field within the same table (you'll see why later)
have a field automatically changed to another row's field value upon
deletion of the referenced field

This is what I've come up with:

create table category (
catid serial primary key,
parentid int references category ( catid ) on delete <?trigger here?>,
catname text unique,
catdesc text
);

I know I need to create a trigger but I'm not sure how to do that.  I've
read the docs, but an example would be great for this particular need.

Also, I know I can do this to update the category with three statements but
I'm pretty sure I can do it in one with a subselect but I'm still fuzzy on
how to do that.  This is a bad mix of pseudo code and sql.

A = category.parentid from row being deleted
B = select parentid from category where catid=A;
update category set parentid=B where parentid=A;
finish deleting row.

Does any of this make sense?

Alan Young
Programmer/Analyst
IDIGlobal.com