SELECT
     IM.PORT_CODE AS '港コード'
    ,IM.PORT_NAME AS '港名'
    , I.AMT AS '入国者数'
    , O.AMT AS '出国者数'
    , I.AMT - O.AMT AS '差分'
FROM
    ( 
        SELECT DISTINCT
            IMMIGRATION.PORT_CODE
            , PORT_NAME 
        FROM
            IMMIGRATION 
            INNER JOIN PORT P 
                ON IMMIGRATION.PORT_CODE = P.PORT_CODE
    ) IM 
    LEFT OUTER JOIN ( 
        SELECT
            IM.PORT_CODE AS PORT_CODE
            , IM.AMT AS AMT
            , IM.KIND_CODE 
        FROM
            IMMIGRATION IM 
            INNER JOIN GRP G 
                ON IM.GROUP_CODE = G.GROUP_CODE 
        WHERE
            IM.KIND_CODE = '110' 
            AND G.GROUP_CODE = '120'
    ) I 
        ON IM.PORT_CODE = I.PORT_CODE 
    LEFT OUTER JOIN ( 
        SELECT
            IM.PORT_CODE AS PORT_CODE
            , IM.AMT AS AMT
            , IM.KIND_CODE 
        FROM
            IMMIGRATION IM 
            INNER JOIN GRP G 
                ON IM.GROUP_CODE = G.GROUP_CODE 
        WHERE
            IM.KIND_CODE = '120' 
            AND G.GROUP_CODE = '120'
    ) O 
        ON IM.PORT_CODE = O.PORT_CODE
    WHERE
    I.AMT IS NOT NULL 
    ORDER BY
    (I.AMT - O.AMT) DESC, IM.PORT_CODE DESC