Testa den här frågan (redigerad ) -
SELECT c.*,
CASE c.comment_type WHEN 8 THEN p1.photo_p_id WHEN 17 THEN p2.photo_id ELSE NULL END photo_id,
CASE c.comment_type WHEN 8 THEN p1.column1 WHEN 17 THEN p2.column1 ELSE NULL END column1
FROM comments c
LEFT JOIN photos p1
ON c.object_id = p1.photo_p_id
LEFT JOIN photos p2
ON c.object_id = p2.photo_id
Ytterligare en variant -
SELECT c.*, p.*
FROM comments c
JOIN photos p
ON c.comment_type = 8 AND c.object_id = p.photo_p_id OR c.comment_type = 17 AND c.object_id = p.photo_id