SELECT IM.PORT_CODE AS 港コード
       ,PO.PORT_NAME AS 港名
       ,IMIN.AMT AS 入国者数
       ,IM.AMT AS 出国者数
       ,(IMIN.AMT-IM.AMT) AS 差分
FROM IMMIGRATION AS IM
LEFT JOIN PORT AS PO
       ON IM.PORT_CODE=PO.PORT_CODE
INNER JOIN IMMIGRATION AS IMIN
        ON IMIN.PORT_CODE = IM.PORT_CODE
        AND IMIN.GROUP_CODE = IM.GROUP_CODE
        AND IMIN.KIND_CODE='110'
        AND IMIN.GROUP_CODE='120'
WHERE IM.KIND_CODE='120'
AND IM.GROUP_CODE='120'
AND 差分>0
GROUP BY IM.PORT_CODE
ORDER BY 差分 DESC,港コード DESC