Question

I have the following tables:

content: content_id, user_id, post_date

comment: comment_id, user_id, comment_date

How can I get a count of active unique users grouped by month and year?
Active would mean that the user either posted or commented in that given month.

Answer 1

SELECT
  T.action_year,
  T.action_month,
  COUNT(T.user_id) active_users
FROM
  (
  SELECT DISTINCT user_id, YEAR(post_date) action_year, MONTH(post_date) action_month FROM content
  UNION
  SELECT DISTINCT user_id, YEAR(comment_date) action_year, MONTH(comment_date) action_date FROM comment
  ) T
GROUP BY
  T.action_year,
  T.action_month
ORDER BY
  T.action_year ASC,
  T.action_month ASC