SUM() for strings

[kromol]

Hi,
Is it possible to write function like SUM() but for strings?
I have a table categories with fields id, name, parent_id
and I need a query something like this:

select *, STR_SUM(categories.name) as cat_name from categories
        group by categories.parent_id

where STR_SUM() - a function for string concatenation.
It has to return a list like this: 'cat1, cat2, cat3'
Thanks!