SELECT
IM1.PORT_CODE AS "港コード",
PO.PORT_NAME AS "港名",
IM1.AMT AS "入国者数",
IM2.AMT AS "出国者数",
(IM1.AMT - IM2.AMT) AS "差分"
FROM
IMMIGRATION AS IM1
-- 出国
JOIN
(SELECT
 PORT_CODE,
GROUP_CODE,
 AMT
 FROM
  IMMIGRATION
 WHERE
  KIND_CODE = '120'
 ) AS IM2
ON IM1.GROUP_CODE = IM2.GROUP_CODE
AND IM1.PORT_CODE = IM2.PORT_CODE
JOIN
 PORT PO
ON IM1.PORT_CODE = PO.PORT_CODE

WHERE
IM1.GROUP_CODE = '120'
AND IM1.KIND_CODE = '110'
AND IM1.AMT > IM2.AMT
ORDER BY
差分 DESC,
港コード DESC