What are all the directors excluding NULL?
select distinct director from movie where director != "null"
What about the highest rating their movies have ever received?
select t2.director, max(t1.stars) from rating as t1 join movie as t2 on t1.mid = t2.mid where director != "null" group by director
Also give me those movies' title.
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