instruction: how many NW flights leave WASHINGTON DC
response: SELECT DISTINCT COUNT(*) FROM flight WHERE (airline_code = 'NW' AND from_airport IN (SELECT AIRPORT_SERVICEalias0.airport_code FROM airport_service AS AIRPORT_SERVICEalias0 WHERE AIRPORT_SERVICEalias0.city_code IN (SELECT CITYalias0.city_code FROM city AS CITYalias0 WHERE (CITYalias0.city_name = 'WASHINGTON' AND CITYalias0.state_code IN (SELECT STATEalias0.state_code FROM state AS STATEalias0 WHERE STATEalias0.state_code = 'DC')))))