How to structure database for unread item alerts per user

Question

I just have a general database theory question. I have a need to make something similar to showing what posts/items a user has viewed or not (such as in a forum) or an unread em

Answer 1

Just create a simple cross-reference table (read_posts or something):

user_id|post_id
----------------
2      | 132
53     | 43
....

Make sure that both of these columns are indexed (especially important that the user_id be indexed) and then use a join (or a sub-query) to select unread posts for the logged in user. If you're just trying to show a list of unread posts, for example, you just run:

SELECT * FROM `posts` WHERE `post_id` NOT IN (
    SELECT `post_id` FROM `read_posts` WHERE `user_id`='[$USER ID]')
ORDER BY [your ordering clause]