SELECT
    *,
    入国者数 - 出国者数 AS 差分
FROM
    (
    SELECT
        PORT.PORT_CODE AS 港コード,
        PORT_NAME AS 港名,
        SUM(CASE KIND_CODE WHEN '110' THEN AMT ELSE 0 END) AS 入国者数,
        SUM(CASE KIND_CODE WHEN '120' THEN AMT ELSE 0 END) AS 出国者数
    FROM
        GRP
    INNER JOIN
        IMMIGRATION
    ON
        GRP.GROUP_CODE =  IMMIGRATION.GROUP_CODE
    INNER JOIN
        PORT
    ON
        PORT.PORT_CODE =  IMMIGRATION.PORT_CODE
    WHERE
        GRP.GROUP_CODE = '120'
    GROUP BY
        港コード, 港名
    )
WHERE
    入国者数 > 出国者数
ORDER BY
    差分 DESC, 港コード DESC
;