CREATE TEMPORARY TABLE TMP AS
SELECT 
    PORT.PORT_CODE AS 港コード,
    PORT.PORT_NAME AS 港名,
    SUM( CASE IMMIGRATION.KIND_CODE WHEN '110' THEN  IMMIGRATION.AMT ELSE 0  END ) AS 入国者数,
    SUM( CASE IMMIGRATION.KIND_CODE WHEN '120' THEN  IMMIGRATION.AMT ELSE 0  END ) AS 出国者数
FROM PORT
INNER JOIN
    IMMIGRATION
ON
    PORT.PORT_CODE = IMMIGRATION.PORT_CODE
WHERE
    IMMIGRATION.GROUP_CODE = 120
GROUP BY
    港コード
;

SELECT 
    *,
    入国者数 - 出国者数 AS 差分
FROM 
    TMP
WHERE
    差分 > 0
ORDER BY
    差分 DESC,
    港コード DESC
;