--種別コードは、'110'(入国者) 、'120'(出国者)となる。
SELECT
    I.PORT_CODE	AS 港コード
,   P.PORT_NAME	AS 港名
,   SUM(CASE WHEN I.KIND_CODE = '110' THEN I.AMT ELSE 0 END)	AS 入国者数 --外国人入国者のAMT
,   SUM(CASE WHEN I.KIND_CODE = '120' THEN I.AMT ELSE 0 END)	AS 出国者数 --外国人出国者のAMT
,   SUM(CASE WHEN I.KIND_CODE = '110' THEN I.AMT ELSE 0 END) - 
    SUM(CASE WHEN I.KIND_CODE = '120' THEN I.AMT ELSE 0 END)	AS 差分
FROM
    IMMIGRATION I
    INNER JOIN PORT P
        ON I.PORT_CODE = P.PORT_CODE
    INNER JOIN GRP G
        ON I.GROUP_CODE = G.GROUP_CODE
GROUP BY
    I.PORT_CODE
,   P.PORT_NAME
HAVING 
    差分 >= 0
ORDER BY
    差分 DESC
,   I.PORT_CODE DESC