Något liknande kanske:
SELECT *
FROM bus_route b1
LEFT JOIN bus_route b2 ON b1.from_city=b2.to_city AND b1.to_city=b2.from_city
WHERE b2.from_city IS NULL
Något liknande kanske:
SELECT *
FROM bus_route b1
LEFT JOIN bus_route b2 ON b1.from_city=b2.to_city AND b1.to_city=b2.from_city
WHERE b2.from_city IS NULL