MYSQL Select As Case when number of rows statement

Question

I'm trying to find the number of notifications for each user, I am having a little problem, I had the query working perfect for what I needed it for, then I changed my table around just a little bit.

Working query:

$numNotifications = mysql_num_rows(mysql_query("
SELECT  
      N.*, 
      P.*
      FROM 
          notifications N, 
          posts P
      WHERE 
          N.userID='$session' 
      AND 
          (
          N.action='1' OR N.action='2'
          ) 
      AND 
           N.uniqueID=P.id AND  P.state='0'"
));

However, uniqueID is now different for some rows. when N.aciton is "1" then N.uniqueID should be compared to P.id, however when N.action is "2" it should compare a row in that table with P.id.

Example query, (that SHOULD work, but doesn't)

$numNotifications = mysql_num_rows(mysql_query("
SELECT  
      N.*, 
      P.*, 
      C.*, 
     (CASE WHEN (
                  N.action = 2 AND N.state = 0
                 )
            THEN 
                 C.postID ELSE N.uniqueID END
      ) AS postId  
      FROM 
          notifications N, 
          posts P, 
          comments C 
      WHERE 
          N.userID='$session' 
      AND 
          (
          N.action='1' OR N.action='2'
          ) 
      AND 
           postId=P.id AND  P.state='0'"
));

diagram of my 3 table structures:

http://i41.tinypic.com/nyzolg.png

Answer 1

here you go :)

SELECT
    COUNT(`notif_id`) AS `number_of_new_notifications`
FROM
    (
        SELECT
            `notifications`.`id` AS `notif_id`
        FROM
            `notifications`
        JOIN
            `posts`
        ON
            `notifications`.`uniqueID`=`posts`.`id`
        WHERE
            `notifications`.`userID`='$session'
        AND
            `notifications`.`action`=1
        AND
            `notifications`.`state`=0
        AND
            `posts`.`state`=0
        UNION ALL
        SELECT
            `notifications`.`id` AS `notif_id`
        FROM
            `notifications`
        JOIN
            `comments`
        ON
            `notifications`.`uniqueID`=`comments`.`id`
        JOIN
            `posts`
        ON
            `comments`.`postID`=`posts`.`id`
        WHERE
            `notifications`.`userID`='$session'
        AND
            `notifications`.`action`=2
        AND
            `notifications`.`state`=0
        AND
            `comments`.`state`=0
        AND
            `posts`.`state`=0
    ) AS notification_ids;