MYSQL Select As Case when number of rows statement - mysql

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;