SELECT IM1.PORT_CODE AS 港コード,
       PORT.PORT_NAME AS 港名,
       SUM(IM1.AMT) AS 入国者数,
       SUM(IM2.AMT) AS 出国者数,
       SUM(IM1.AMT) - SUM(IM2.AMT) AS 差分
FROM IMMIGRATION AS IM1
     INNER JOIN IMMIGRATION AS IM2
     ON IM1.PORT_CODE = IM2.PORT_CODE
     AND IM2.KIND_CODE = '120'
     INNER JOIN PORT
     ON IM1.PORT_CODE = PORT.PORT_CODE
     INNER JOIN GRP
     ON IM1.GROUP_CODE = GRP.GROUP_CODE
WHERE IM1.KIND_CODE = '110'
      AND GRP.GROUP_NAME = '外国人'
GROUP BY 港コード
HAVING SUM(IM1.AMT) - SUM(IM2.AMT) > 0
ORDER BY 差分 DESC, 港コード DESC