SQL查询：期望多个结果，但只返回一个

Question

运行此查询后，我得到的结果数量有问题：

SELECT g.genre_name, 
       count(distinct(d.director_id))
  FROM genre g, 
       director d, 
       movie m
 WHERE g.genre_id=m.genre_id 
       AND m.director_id=d.director_id;   

表及其键是：

1. genre(genre_id, genre_name)
2. movie(genre_id, director_id)
3. director(director_id)

有人可以帮忙吗？提前谢谢！

Answer 1

尝试添加GROUP BY

  SELECT g.genre_name, 
         count(distinct(d.director_id))
    FROM genre g, 
         director d, 
         movie m
   WHERE g.genre_id=m.genre_id 
         AND m.director_id=d.director_id
GROUP BY g.genre_name;