User Engagement Histogram

UserEngagementHistogram.sql

SELECT active.num_days_active
, COUNT(DISTINCT active.user_id) AS num_users -- get how many users have a number of active days
FROM (
SELECT a.user_id, COUNT(DISTINCT a.created_at) AS num_days_active
FROM activity a
GROUP BY a.user_id) AS active
LEFT JOIN activity a2 ON a2.user_id = active.user_id
WHERE a2.created_at >= CURRENT_DATE - 28
GROUP BY active.num_days_active
ORDER BY active.num_days_active ASC;