How many companies operate in airports in the United States?
SELECT count ( distinct T1.id )  FROM operate_company AS T1 JOIN flight AS t2 ON T1.id  =  T2.company_id JOIN airport AS T3 ON T2.airport_id  =  T3.id where T3.country  =  "United States"
companies operate in airports refers to joining operate_company with flight and airport tables on respective IDs; in the United States refers to where airport.country = \"United States\"; counting companies refers to COUNT(DISTINCT T1.id);