query with several LEFT JOIN , not getting desired result - mysql

Question

Here's a sql fiddle
Can anyone tell me how can I get this output using LEFT JOIN?
notification_recipient pm_sender msg modification_page_id
Peter Tom Hello NULL
notification_recipient pm_sender msg modification_page_id
Peter NULL NULL 2
Here's the query that I have tried:
SELECT u.name AS notification_recipient,us.name AS pm_sender,
p.msg,um.page_id AS modification_page_id
FROM notification n
LEFT JOIN pm p ON p.pm_id = n.pm_id
LEFT JOIN users u ON u.user_id = p.recipent_id
LEFT JOIN users us ON us.user_id = p.sender_id
LEFT JOIN user_modification um ON um.modification_id = n.modification_id
WHERE u.name = 'Peter'
AND n.is_read = '0'
I was looking for some sort conditional join, which means joining different tables based on whether a value exists in a field, but I couldn't find one that would work on my example. Any other efficient solution would also be appreciated.
Background:
I'm planning to make a notification system that sends out different types of messages to users (private messages among users, message that their modifications to an entry has been approved etc).
When a user logs in, I want to make a query to find out if there's any unread notifications for that user. If yes, a notification will be sent to him via Ajax.
To illustrate, suppose that Tom has sent a private message to Peter and his modification to an entry is approved, two triggers from table pm and user_modification will be invoked to add two new rows into notification. Column pm_id is referenced by pm and modification_id is by modification. is_read is defaulted 0 as Not Read.
Here's the table schema:
CREATE TABLE notification
(`id` int, `modification_id` int,`pm_id` int,`is_read` int)
;
INSERT INTO notification
(`id`,`modification_id`,`pm_id`,`is_read`)
VALUES
(1,1,NULL,0),
(2,NULL,1,0),
(3,2,NULL,0)
;
CREATE TABLE user_modification
(`modification_id` int, `user_id` int,`page_id` int, `is_approved` int)
;
INSERT INTO user_modification
(`modification_id`,`user_id`,`page_id`,`is_approved`)
VALUES
(1,1,5,1),
(2,2,2,1),
(3,3,3,0)
;
CREATE TABLE pm
(`pm_id` int, `sender_id` int,`recipent_id` int,`msg` varchar(200))
;
INSERT INTO pm
(`pm_id`,`sender_id`,`recipent_id`,`msg`)
VALUES
(1,1,2,'Hello');
CREATE TABLE users
(`user_id` int, `name`varchar(20))
;
INSERT INTO users
(`user_id`,`name`)
VALUES
(1,'Tom'),
(2,'Peter'),
(3,'David')
;
Here's the output I want for a notification if user David logs in. Each row for each message type.
notification_recipient pm_sender msg modification_page_id
Peter Tom Hello NULL
notification_recipient pm_sender msg modification_page_id
Peter NULL NULL 2
The notification to Peter will be like this:
`1.You have received a private message from Tom.
2.your modification on <a href='mysite.com/5'>that page</a> is approved`.

Answer 1

This query should do the job. Here's SQLFiddle
SELECT
n.id,
IF(pmu.name IS NULL, pmm.name, pmu.name) recipient,
pmus.name sender, pm.msg, m.modification_id
FROM
notification n
LEFT JOIN user_modification m ON (n.modification_id = m.modification_id)
LEFT JOIN pm ON (n.pm_id = pm.pm_id)
LEFT JOIN users pmu ON (pm.recipent_id = pmu.user_id)
LEFT JOIN users pmus ON (pm.sender_id = pmus.user_id)
LEFT JOIN users pmm ON (m.user_id = pmm.user_id)
WHERE
(pmu.name = 'Peter' OR
pmm.name = 'Peter') AND
n.is_read = 0;