instruction: how much is an economy ticket from WASHINGTON to SAN FRANCISCO on 9 30
response: SELECT DISTINCT fare.fare_id FROM airport_service AS AIRPORT_SERVICE_0, airport_service AS AIRPORT_SERVICE_1, city AS CITY_0, city AS CITY_1, date_day AS DATE_DAY_0, date_day AS DATE_DAY_1, days AS DAYS_0, days AS DAYS_1, fare, fare_basis AS FARE_BASIS_0, fare_basis AS FARE_BASIS_1, flight, flight_fare WHERE ((CITY_1.city_code = AIRPORT_SERVICE_1.city_code AND CITY_1.city_name = 'SAN FRANCISCO' AND DATE_DAY_1.day_number = 30 AND DATE_DAY_1.month_number = 9 AND DATE_DAY_1.year = 1991 AND DAYS_1.day_name = DATE_DAY_1.day_name AND flight.flight_days = DAYS_1.days_code AND flight.to_airport = AIRPORT_SERVICE_1.airport_code) AND CITY_0.city_code = AIRPORT_SERVICE_0.city_code AND CITY_0.city_name = 'WASHINGTON' AND DATE_DAY_0.day_number = 30 AND DATE_DAY_0.month_number = 9 AND DATE_DAY_0.year = 1991 AND DAYS_0.day_name = DATE_DAY_0.day_name AND FARE_BASIS_1.basis_days = DAYS_0.days_code AND fare.fare_basis_code = FARE_BASIS_1.fare_basis_code AND flight_fare.fare_id = fare.fare_id AND flight.flight_id = flight_fare.flight_id AND flight.from_airport = AIRPORT_SERVICE_0.airport_code) AND FARE_BASIS_0.economy = 'YES' AND fare.fare_basis_code = FARE_BASIS_0.fare_basis_code