Question

I have two MySQL tables describing data that can be extended into subclasses, one that describes the parent class data and another one describing metadata fields as one-to-many relationships.

Example rows from table page:

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

Example rows from table page_metadata:

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

Question: Is there some way to fetch all this data in this format?

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