I came up with the exact solution,except my JOINs are in a different order. Using both my solution and copy pastying this solution I get the same error:

Column 'hackers.name' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Could someone please shed some light into this? Also, is the "c.difficulty_level = d.difficulty_level" in the WHERE clause neccessary? Isn't that "taken care of" from the JOIN condition?

EDIT: Changing to MySQL resulted in a working code.