Joining different tables based on column value

Question

I have a table called notifications:

CREATE TABLE `notifications` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(11) DEFAULT         


        

Answer 1

I'm not 100% sure the syntax is right and I have no chance to test it right now, but the idea should be clear.

SELECT DISTINCT n.id 
FROM notifications n 
JOIN (
     (SELECT b.id, 'book' AS type FROM books b WHERE b.is_visible = 1)
  UNION
     (SELECT i.id, 'interview' AS type FROM interviews i WHERE i.is_visible = 1)
) ids ON n.parent_id = ids.id AND n.parent_type = ids.type
WHERE n.user_id = 1