gistfile1.sql

#Table
CREATE TABLE `departments` (
  `in` int(11) unsigned NOT NULL,
  `out` int(11) unsigned NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1

#Via JOIN
SELECT i.count + o.count as total, i.in
FROM
	(
		SELECT COUNT(*) count, `in`
		FROM departments
		GROUP BY `in`) i
	LEFT JOIN 
		(SELECT COUNT(*) count, `out`
		FROM departments
		GROUP BY `out`) o ON i.in = o.out
ORDER BY total DESC LIMIT 1


#Via UNION
SELECT SUM(count) total, airport
FROM 
	((SELECT COUNT(*) count, `in` as airport
	FROM departments
	GROUP BY `in`) i
	UNION ALL
	(SELECT COUNT(*) count, `out` as airport
	FROM departments
	GROUP BY `out`) o) t
GROUP BY airport
ORDER BY total DESC 
LIMIT 1