Calculate the average duration travelled by subscribers that both started and ended their trip in Mountain View City Hall and indicate the date when the station was first installed.
average duration = DIVIDE(SUM(duration), COUNT(id)); subscribers refers to subscription_type = 'subscriptions'; started and ended their trip at Mountain View City Hall refers to start_station_name = 'Mountain View City Hall' and end_station_name = 'Mountain View City Hall'; when the station was first installed refers to installation_date;
SELECT AVG(T1.duration), T2.installation_date FROM trip AS T1 INNER JOIN station AS T2 ON T2.name = T1.start_station_name WHERE T1.start_station_name = 'Mountain View City Hall' AND T1.subscription_type = 'Subscriber' AND T1.end_station_name = 'Mountain View City Hall'