Question: What is the percentage ration of customers to subscribers that started their trips within the city of San Francisco?
SQL: 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'