SELECT PORT_CODE AS '港コード', PORT_NAME AS '港名', inamt AS '入国者数', outamt AS '出国者数', subs AS '差分'
FROM (SELECT *, inamt - outamt AS subs
FROM (SELECT PORT_CODE, AMT AS inamt
      FROM IMMIGRATION
      WHERE KIND_CODE = '110') AS n
         JOIN (SELECT PORT_CODE, AMT AS outamt
               FROM IMMIGRATION
               WHERE KIND_CODE = '120') AS s
              USING(PORT_CODE)
         JOIN PORT USING(PORT_CODE)
WHERE inamt > outamt
)
ORDER BY subs DESC, PORT_CODE DESC