Joining different tables based on column value

Joining different tables based on column value

I have a table called notifications:
CREATE TABLE `notifications` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`user_id` int(11) DEFAULT NULL,
`type` varchar(20) NOT NULL DEFAULT '',
`parent_id` int(11) DEFAULT NULL,
`parent_type` varchar(15) DEFAULT NULL,
`type_id` int(11) DEFAULT NULL,
`etc` NULL
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=25 DEFAULT CHARSET=utf8;
Each notification is related to a different table, the value parent_type
field specifies the name of the table that I want to * join the table
with. All target tables have several similar columns:
CREATE TABLE `tablename` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`is_visible` tinyint(1) NOT NULL,
`etc` NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
Currently I'm using this query for selecting notifcations that their
related row in the target table exist and their is_visible field is 1:
SELECT n.id,
FROM notifications n
LEFT JOIN books b ON n.parent_id = b.id AND n.parent_type = 'book' AND
b.is_visible = 1
LEFT JOIN interviews i ON n.parent_id = i.id AND n.parent_type =
'interview' AND i.is_visible = 1
LEFT JOIN other tables...
WHERE n.user_id = 1
GROUP BY n.id
which since it is a left join it returns the notification if it matches
any table or not, how can I rewrite it so it doesn't return notifications
that don't match with any row in the target table? I have also tried the
CASE statement unsuccessfully.