Task: Text to Code
Input (English): Step 1: For each row in airlines table, find the corresponding rows in flights table.
Step 2: find FlightNo of the results of step 1 whose airlines's Airline equals United Airlines and Country equals USA
Output (English): SELECT T2.FlightNo FROM airlines AS T1 JOIN flights AS T2 WHERE T1.Airline = "United Airlines" AND T1.Country = "USA"