SELECT
  jpn.港コード,
  jpn.港名,
  jpn.入国者数,
  for.出国者数,
  jpn.入国者数 - for.出国者数 AS 差分
FROM
  (
    SELECT
      imm.PORT_CODE AS 港コード,
      port.PORT_NAME AS 港名,
      imm.AMT AS 入国者数
    FROM
      IMMIGRATION AS imm
    INNER JOIN
      PORT AS port
    ON
      imm.PORT_CODE = port.PORT_CODE
    WHERE
      imm.KIND_CODE = '110' 
  ) AS jpn
INNER JOIN
  (
    SELECT
      imm.PORT_CODE AS 港コード,
      port.PORT_NAME AS 港名,
      imm.AMT AS 出国者数
    FROM
      IMMIGRATION AS imm
    INNER JOIN
      PORT AS port
    ON
      imm.PORT_CODE = port.PORT_CODE
    WHERE
      imm.KIND_CODE = '120'
  ) AS for
ON
  jpn.港コード = for.港コード
AND
  jpn.港名 = for.港名
AND 
  for.出国者数 < jpn.入国者数
ORDER BY
  差分 DESC,
  jpn.港コード DESC