Question: 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.
SQL: 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'