子查询 - 查找每张专辑的平均曲目数

Question

我正在尝试编写一个子查询来加入“专辑”和“曲目”表。

最终我需要弄清楚专辑中平均有多少首歌曲名称中带有“摇滚”一词。所选专辑必须至少包含八首歌曲。

SELECT AVG(tr.track_id)
FROM(SELECT al.album_id AS album,
            tr.name,
            COUNT(tr.track_id)
     FROM track as tr
     LEFT OUTER JOIN album as al ON al.album_id = tr.album_id
     WHERE tr.name LIKE '%Rock%'
     GROUP tr.name )AS ag
     HAVING COUNT(al.album_id) >= 8;
     

Answer 1

你很接近。

HAVING

子句应该在内部查询中，仅过滤其中包含超过 8 首歌曲的歌曲。你也按曲目名称而不是专辑名称过滤，所以我也改变了它。

SELECT AVG(ag.cnt)
FROM(SELECT al.album_id AS id,
            COUNT(*) as cnt
     FROM track as tr
     JOIN album as al ON al.album_id = tr.album_id
     WHERE al.name LIKE '%Rock%'
     GROUP al.album_id
     HAVING COUNT(al.album_id) >= 8 ) AS ag

Answer 2

SELECT AVG(ag.count_track)
FROM (
  SELECT 
     a.title,
     COUNT(t.track_id) as count_track
  FROM track AS t
  LEFT OUTER JOIN album AS a ON a.album_id = t.album_id
  WHERE a.title LIKE '%Rock%'
  GROUP BY a.title
) AS ag
WHERE count_track >= 8