Update table based on another table

I'm trying to update a column in a table based on another column in another table.

UPDATE eval e
   SET rank = (SELECT p.desc
                 FROM Position p
                WHERE p.id = e.faculty 
                  AND p.date >= '2011-05-20'
              )

p.id and e.faculty correspond. I want to update rank with p.desc if the id's are the same. (e.faculty and p.id)

Any help will be great! :)