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

Based on this description I would use a simple table with maybe 3 columns.

1. User ID
2. Post ID
3. Timestamp First Viewed

When a user views a post, add a row to the table. If a row does not exist in the table for a given user/post id combo, then they have not viewed the post.

Answer 2

While reviewing the relevant schema for phpBB, I found the following:

# Table: 'phpbb_topics_track'
CREATE TABLE phpbb_topics_track (
    user_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL,
    topic_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL,
    forum_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL,
    mark_time int(11) UNSIGNED DEFAULT '0' NOT NULL,
    PRIMARY KEY (user_id, topic_id),
    KEY topic_id (topic_id),
    KEY forum_id (forum_id)
) CHARACTER SET `utf8` COLLATE `utf8_bin`;

And:

# Table: 'phpbb_forums_track'
CREATE TABLE phpbb_forums_track (
    user_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL,
    forum_id mediumint(8) UNSIGNED DEFAULT '0' NOT NULL,
    mark_time int(11) UNSIGNED DEFAULT '0' NOT NULL,
    PRIMARY KEY (user_id, forum_id)
) CHARACTER SET `utf8` COLLATE `utf8_bin`;

Then I look here in their wiki:

This table keeps record for visited topics in order to mark them as read or unread. We use the mark_time timestamp in conjunction with last post of topic x's timestamp to know if topic x is read or not.

In order to accurately tell whether a topic is read, one has to also check phpbb_forums_track.

So essentially they have a lookup table to store the data associated with a user's viewing of a topic (thread), and then check it against the timestamp in the forum view table, to determine whether the topic has been viewed by the user.

Answer 3

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]