SELECT
  PORT_CODE AS '港コード',
  PORT_NAME AS '港名',
  `入国者数`,
  `出国者数`,
  (`入国者数` - `出国者数`) AS '差分'
FROM (
  SELECT
    PORT.PORT_CODE,
    PORT_NAME,
    SUM(CASE IMMIGRATION.KIND_CODE
      WHEN '110' THEN AMT ELSE 0 END
    ) AS '入国者数',
    SUM(CASE IMMIGRATION.KIND_CODE
      WHEN '120' THEN AMT ELSE 0 END
    ) AS '出国者数'
  FROM
    PORT
    JOIN IMMIGRATION
      ON PORT.PORT_CODE = IMMIGRATION.PORT_CODE
    JOIN GRP
      ON IMMIGRATION.GROUP_CODE = GRP.GROUP_CODE
  WHERE
    GRP.GROUP_CODE = '120'
  GROUP BY
    PORT.PORT_CODE, PORT_NAME
)
ORDER BY
  `差分` DESC,
  `港コード` DESC