I don't know if this is even worthwhile but is it possible to only display items from a db that a particular user has not seen before?

I have a content management system where users have their own profile area. I just made a little announcement feature where I can post announcements to all users or just 1. Instead of listing all the announcements every time a person visits, is it possible to display unseen one's only?

A field $lastlogin gets updated every time a user logs in so I was thinking of comparing $lastlogin to the announcement date to display new messages - but I'm not sure if this is the right way to go.

Any input is greatly appreciated!