SELECT main.PORT_CODE AS 港コード
      ,main.PORT_NAME AS 港名
      ,sub_In.AMT      AS 入国者数
      ,sub_Out.AMT     AS 出国者数   
      ,sub_In.AMT - sub_Out.AMT AS 差分 
FROM PORT main
INNER JOIN (
SELECT a.PORT_CODE
      ,MAX(a.KIND_CODE) AS KIND_CODE
      ,MAX(a.AMT) AS AMT
FROM IMMIGRATION a
INNER JOIN GRP b
ON a.GROUP_CODE = b.GROUP_CODE
AND b.GROUP_NAME = '外国人'
WHERE a.KIND_CODE = '110'
GROUP BY a.PORT_CODE
)sub_In 
ON main.PORT_CODE = sub_In.PORT_CODE

INNER JOIN (
SELECT a.PORT_CODE
      ,MAX(a.KIND_CODE) AS KIND_CODE
      ,MAX(a.AMT) AS AMT
FROM IMMIGRATION a
INNER JOIN GRP b
ON a.GROUP_CODE = b.GROUP_CODE
AND b.GROUP_NAME = '外国人'
WHERE a.KIND_CODE = '120'
GROUP BY a.PORT_CODE
)sub_Out 
ON main.PORT_CODE = sub_Out.PORT_CODE

WHERE sub_In.AMT > sub_Out.AMT
ORDER BY (sub_In.AMT - sub_Out.AMT) DESC