For each director, return the director's name together with the title of the movie they directed that received the highest rating among all of their movies, and the value of that rating. Ignore movies whose director is NULL.
select t2.title, t1.stars, t2.director, max(t1.stars) from rating as t1 join movie as t2 on t1.mid = t2.mid where director != "null" group by director