https://www.youtube.com/watch?v=UW02m1Hkt3A

DROP TABLE IF EXISTS Passenger

CREATE TABLE Passenger
(
 A varchar(10),
 B varchar(10),
 PassengerCount int
)

INSERT INTO Passenger
VALUES
('Delhi','Agra',500),
('Agra','Delhi',350),
('Guwahati','Kolkata',300),
('Kolkata','Guwahati',200),
('Bengaluru','Chennai',1000),
('Chennai','Bengaluru',500)

SELECT * FROM Passenger

CASE WHEN

SELECT  
	CASE WHEN A>B THEN A ELSE B END AS A,
    CASE WHEN A<B THEN A ELSE B END AS B,
    SUM(Passengercount) AS TotalCount
FROM Passenger
GROUP BY CASE WHEN A>B THEN A ELSE B END, CASE WHEN A<B THEN A ELSE B END;