Finding out how many users are watching or tracking?

There are “_user” tables.
category_users, topic_users, group_users and tag_users
Each has a notification_level field where the integer indicates a members preference - 0 Muted, 1 Normal, 2 Tracked, 3 Watched

So a query something like this should work

SELECT COUNT(id) 
FROM category_users 
WHERE notification_level = 3 
GROUP BY category_id 

to give you how many are Watching each category.

7 Likes