What are all the names of the pilots who flew a flight in the United States?
SELECT DISTINCT T2.pilot FROM airport AS T1 JOIN flight AS T2 ON T1.id   =   T2.airport_id WHERE T1.country   =   'United States'
pilots who flew a flight in the United States refers to joining tables airport and flight on airport_id and filtering where country = 'United States'; DISTINCT is used to get unique names of pilots.