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