MySQL get top average entries

Question

I am trying to write a mysql query to return the top 3 courses that have the highest average course rating. I have two tables, Ratings and Courses.

The Ratings table:

courseId    rating  
   1           6    
   2           2        
   1           4
   2           5
   3           3
   4           0
   6           0

The Courses Table:

courseId        cnum        cname
   1            100         name1
   2            112         name2
   3            230         name3
   4            319         name4
   5            122         name5
   6            320         name6

I need to return the top 3 courses that have the highest average rating. Any ideas how I could do this? Thanks

Answer 1

SELECT Courses.*
FROM   Courses NATURAL JOIN (
  SELECT   courseId, AVG(rating) avg_rating
  FROM     Ratings
  GROUP BY courseId
  ORDER BY avg_rating DESC
  LIMIT    3
) t

See it on sqlfiddle.