使用连接查询中的字段值作为列

Question

我有两张描述可以扩展到子类的数据的 MySQL 表，一张描述父类数据，另一张将元数据字段描述为一对多关系。

表page中的示例行：

id   | name
-----+-----------------------
123  | Example page
999  | Another page

表page_metadata中的示例行：

page_id | key        | value
--------+------------+----------------
123     | picture    | test.jpg
123     | video      | example.avi
123     | sound      | sound.mp3
999     | picture    | something.jpg

问题是否有某种方法可以以这种格式获取所有这些数据？

id   | name             | picture       | video       | sound
-----+------------------+---------------+-------------+------------------
123  | Example page     | test.jpg      | example.avi | sound.mp3
999  | Another page     | something.jpg | NULL        | NULL

Answer 1

SELECT pm.page_id as id
  , p.name
  , max(IF(pm.key = 'picture', pm.value, null)) as picture
  , max(IF(pm.key = 'video', pm.value, null)) as video
  , max(IF(pm.key = 'sound', pm.value, null)) as sound
FROM page p
INNER JOIN page_metadata pm ON (p.id = pm.page_id)
GROUP BY p.id

Answer 2

尝试一下

SELECT page.id, page.name, picture.value, video.value, sound.value
FROM page
LEFT OUTER JOIN (SELECT page_id, value FROM page_metadata WHERE key = 'picture') as picture
  ON page.id = picture.page_id
LEFT OUTER JOIN (SELECT page_id, value FROM page_metadata WHERE key = 'video') as video
  ON page.id = video.page_id
LEFT OUTER JOIN (SELECT page_id, value FROM page_metadata WHERE key = 'sound') as sound
  ON page.id = sound.page_id

应该会得到你需要的东西