SELECT
    *
FROM
    (
        SELECT
            IMMIGRATION.PORT_CODE AS 港コード,
            PORT.PORT_NAME AS 港名,
            SUM(CASE WHEN IMMIGRATION.KIND_CODE = '110' THEN IMMIGRATION.AMT ELSE 0 END) AS 入国者数,
            SUM(CASE WHEN IMMIGRATION.KIND_CODE = '120' THEN IMMIGRATION.AMT ELSE 0 END) AS 出国者数,
            (SUM(CASE WHEN IMMIGRATION.KIND_CODE = '110' THEN IMMIGRATION.AMT ELSE 0 END) - SUM(CASE WHEN IMMIGRATION.KIND_CODE = '120' THEN IMMIGRATION.AMT ELSE 0 END)) AS 差分
        FROM
            IMMIGRATION
        JOIN
            PORT ON IMMIGRATION.PORT_CODE = PORT.PORT_CODE
        JOIN
            GRP ON IMMIGRATION.GROUP_CODE = GRP.GROUP_CODE
        WHERE
            GRP.GROUP_NAME = "外国人"
        GROUP BY
            IMMIGRATION.PORT_CODE
    ) AS t
WHERE
    t.差分 > 0
ORDER BY
    t.差分 DESC,
    t.港コード DESC