根据另一个表的某些列匹配的总结果从MySQL中选择

Question

I have two tables in my DB

Genre contains: id, genre_id

Music contains: id, song_id, genre_id

I want to show the result of:

SELECT * FROM Genre WHERE total number of rows in Music 
WHERE Genre.genre_id = Music.genre_id > 4

I have been scratching my head for hours trying to get it using JOIN but I couldn't get how logically to solve it then put it in mysql code.

Answer 1

Your desired output is only a couple of minor syntax changes off from a valid query:

SELECT *
  FROM Genre g
 WHERE (SELECT COUNT(*) FROM Music m WHERE g.genre_id = m.genre_id) > 4

While that's probably the most understandable way of doing it, a combination of LEFT OUTER JOIN , GROUP BY and HAVING may be more efficient:

SELECT g.id, g.genre_id
  FROM Genre g
  LEFT OUTER JOIN Music m ON m.genre_id = g.genre_id
 GROUP BY g.id, g.genre_id
HAVING COUNT(*) > 4