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