How to combine UNION and INNER JOIN? I'm fetching the grandparent of all nodes in a tree as follows:

SELECT P.id, P.parent, GP.parent gp_id FROM product_groups P             
INNER JOIN product_groups GP ON P.parent = GP.id


There is no entry with id=0 in the table, but some where parent=0, indicating roots of the trees. Those rows are not fetched by this query, as there's no grandparent to join them with. I'd like those rows returned, with 0 (or NULL) for both parent and gp_id.

I was planning to UNION the second table with (0,0), but can't figure out the syntax. How to do this?
Just do a LEFT JOIN instead:

SELECT P.id, P.parent, GP.parent gp_id FROM product_groups P             
LEFT JOIN product_groups GP ON P.parent = GP.id


That will return all rows from P and any non-matching rows in GP will have NULL values.