title: having trouble with sql about movie theme
body: <pre><code>Movie(mID int, title text, year int, director text);
  Reviewer(rID int, name text);
  Rating(rID int, mID int, stars int, ratingDate date);
  </code></pre>
  
  <p>For each director, return the director's name together with the title(s) of the movie(s) they directed that received the highest rating among all of their movies, and the value of that rating. Ignore movies whose director is NULL.</p>
  
  <p>I am stuck at highest rating part.</p>
  
  <pre><code>select title, director
  from Movie natural join Rating
  where director is not NULL
  </code></pre>
  
  <p>i know i just couldn't really write it.</p>