When did event number 2 happen and how many users were active?
event number refers to event_id; event_id = 2; active refers to is_active = 1;
SELECT COUNT(T1.app_id) AS num FROM app_events AS T1 INNER JOIN events AS T2 ON T1.event_id = T2.event_id WHERE T1.event_id = 2 AND T1.is_active = 1 GROUP BY T2.timestamp