How many trips with a bike borrowed from the stations in San Francisco were made by a subscriber?
bike was borrowed from refers to start_station_id; San Francisco refers to city = 'San Francisco'; subscriber refers to subscription_type = 'Subscriber';
SELECT COUNT(T1.id) FROM trip AS T1 INNER JOIN station AS T2 ON T2.ID = T1.start_station_id WHERE T2.city = 'San Francisco' AND T1.subscription_type = 'Subscriber'