SELECT
a.PORT_CODE AS 港コード
,PORT_NAME AS 港名
,外国人入国者のAMT AS 入国者数
,外国人出国者のAMT AS 出国者数
,ABS(外国人入国者のAMT - 外国人出国者のAMT) AS 差分
FROM PORT a
LEFT JOIN (
SELECT SUM(AMT) AS 外国人入国者のAMT
,PORT_CODE
FROM IMMIGRATION 
WHERE KIND_CODE = '110'
AND GROUP_CODE = '120'
GROUP BY PORT_CODE
)b
on b.PORT_CODE = a.PORT_CODE
LEFT JOIN (
SELECT SUM(AMT) AS 外国人出国者のAMT
,PORT_CODE
FROM IMMIGRATION 
WHERE KIND_CODE = '120'
AND GROUP_CODE = '120'
GROUP BY PORT_CODE
)c
on c.PORT_CODE = a.PORT_CODE

ORDER BY 差分 DESC ,a.PORT_CODE DESC