Database design for Group Notification System

Question

I am trying to create a group notification system. If I am in a group, then anyone who comment on the group's wall, a notification will send out to every group member. Here is my database design: I have two tables: Notification and NotificationRead.

NotificationRead
+userId (String)
+lastRead (int) - default is 0

Notification
...
+time(int)
...
Every user has one entry in NotificationRead, it keep track of when is the last time I read my notification. The logic is: for a particular user, if Notification.time > NotificationRead.lastRead, then that notification is considered unread. Let say that in group A, there are 4 notifications I have not read, and their time is 7, 8, 9, 10, then when I click onto group A, I set my NotificationRead.lastRead = 10 (the largest time), so I wont read them again. New notifications will have their time start at 11. Now, here is my problem. Let say I have 3 groups, A, B and C

A (4): largest time is 10
B (1): largest time is 14
C (1): largest time is 12

if I click onto A, my NotificationRead.lastRead = 10, the 4 next to A clear off, 1 next to B and C stay put. Now if I click on B, my lastRead now is 14, so not only it clear off the 1 next to B but also the 1 next to C since 14 > 12. Can anyone help me think of a way to solve this. I am open to completely redesign everything

Answer 1

Cant you just add a groupID column to your NotificationRead table so you know the lastRead value for each User\Group combination>?

Answer 2

If you wish to know the last notification time per user per group, you must store that information. Therefore, each user must have more than one record in NotificationRead, which must become a separate table from the user table. This table will have three columns, the user_id, the group_id, and the lastread value for that user/group.