Facebook like notifications tracking (DB Design)

Question

I am just trying to figure out how Facebook's database is structured for tracking notifications.

I won't go much into complexity like Facebook is. If we imagine a simple table structure for notificaitons:

notifications (id, userid, update, time);

We can get the notifications of friends using:

SELECT `userid`, `update`, `time`
FROM `notifications`
WHERE `userid` IN 
(... query for getting friends...)

However, what should be the table structure to check out which notifications have been read and which haven't?

Answer 1

I dont know if this is the best way to do this, but since I got no ideas from anyone else, this is what I would be doing. I hope this answer might help others as well.

We have 2 tables

notification
-----------------
id (pk)
userid
notification_type (for complexity like notifications for pictures, videos, apps etc.)
notification
time


notificationsRead
--------------------
id (pk) (i dont think this field is required, anyways)
lasttime_read
userid

The idea is to select notifications from notifications table and join the notificationsRead table and check the last read notification and rows with ID > notificationid. And each time the notifications page is opened update the row from notificationsRead table.

The query for unread notifications I guess would be like this..

SELECT `userid`, `notification`, `time` from `notifications` `notificationsRead`
WHERE 
`notifications`.`userid` IN ( ... query to get a list of friends ...) 
AND 
(`notifications`.`time` > (
    SELECT `notificationsRead`.`lasttime_read` FROM `notificationsRead` 
    WHERE `notificationsRead`.`userid` = ...$userid...
))

The query above is not checked. Thanks to the idea of db design from @espais