What is the percentage ration of customers to subscribers that started their trips within the city of San Francisco?
customer refers to subscription_type = 'customer'; subscriber refers to subscription_type = 'subscriber'; started their trips within refers to start_station_id; percentage ratio = DIVIDE(SUM(subscription_type = 'Customer'), SUM(subscription_type = 'Subscriber')) as percentage;
SELECT CAST(SUM(CASE WHEN T1.subscription_type = 'Customer' THEN 1 ELSE 0 END) AS REAL) * 100 / SUM(CASE WHEN T1.subscription_type = 'Subscriber' THEN 1 ELSE 0 END) FROM trip AS T1 LEFT JOIN station AS T2 ON T2.name = T1.start_station_name WHERE T2.city = 'San Francisco'