SELECT
    IM1.PORT_CODE AS '港コード',
    PR.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
    (SELECT *
    FROM IMMIGRATION
    WHERE GROUP_CODE = (
        SELECT GROUP_CODE
        FROM GRP
        WHERE GROUP_NAME = '外国人'
        )
        AND KIND_CODE = '110'
    ) AS IM1
JOIN
    (SELECT *
    FROM IMMIGRATION
    WHERE GROUP_CODE = (
        SELECT GROUP_CODE
        FROM GRP
        WHERE GROUP_NAME = '外国人'
        )
        AND KIND_CODE = '120'
    ) AS IM2
ON
    IM1.PORT_CODE = IM2.PORT_CODE
JOIN
    PORT AS PR
ON
    PR.PORT_CODE = IM1.PORT_CODE
GROUP BY
    IM1.PORT_CODE
HAVING
    SUM(IM1.AMT) > SUM(IM2.AMT)
ORDER BY
    差分 DESC,
    港コード DESC