dirty.sql

SELECT uts.user_id, MIN(uts.ts) as earliest_action FROM (
  SELECT user_id, MIN(placed) as ts FROM ADS WHERE user_id > 0 GROUP BY user_id UNION ALL
  SELECT user_id, MIN(placed_on) as ts FROM reviews WHERE user_id > 0 GROUP BY user_id UNION ALL
  SELECT EVENTS_user_xref.user_id as user_id, MIN(EVENTS.placed_time) as ts FROM EVENTS JOIN EVENTS_user_xref ON EVENTS.id = EVENTS_user_xref.event_id WHERE EVENTS_user_xref.user_id > 0 AND EVENTS.placed_time > 0 GROUP BY EVENTS_user_xref.user_id UNION ALL
  SELECT EVENTS_user_xref.user_id as user_id, MIN(EVENTS_archive.placed_time) as ts FROM EVENTS_archive JOIN EVENTS_user_xref ON EVENTS_archive.id = EVENTS_user_xref.event_id WHERE EVENTS_user_xref.user_id > 0 AND EVENTS_archive.placed_time > 0 GROUP BY EVENTS_user_xref.user_id UNION ALL
  SELECT user_id, UNIX_TIMESTAMP(MIN(time)) as ts FROM user_action_log WHERE time > 0 AND user_id > 0 GROUP BY user_id UNION ALL
  SELECT user_id, MIN(time) as ts FROM email_mask_log WHERE time > 0 AND user_id > 0 GROUP BY user_id
) uts
GROUP BY user_id