SELECT p.PORT_CODE AS '港コード'
  , PORT_NAME AS '港名'
  , i.AMT AS '入国者数'
  , e.AMT AS '出国者数'
  , i.AMT - e.AMT AS '差分'
FROM PORT p
INNER JOIN (
  SELECT PORT_CODE, AMT
  FROM IMMIGRATION im
    INNER JOIN GRP g ON im.GROUP_CODE = g.GROUP_CODE
  WHERE im.KIND_CODE = '110' AND g.GROUP_NAME = '外国人'
  ) i ON i.PORT_CODE = p.PORT_CODE
INNER JOIN (
  SELECT PORT_CODE, AMT
  FROM IMMIGRATION im
    INNER JOIN GRP g ON im.GROUP_CODE = g.GROUP_CODE
  WHERE im.KIND_CODE = '120' AND g.GROUP_NAME = '外国人'
  ) e ON e.PORT_CODE = p.PORT_CODE
WHERE i.AMT > e.AMT
ORDER BY i.AMT - e.AMT DESC, p.PORT_CODE