select *,
入国者数 - 出国者数 as '差分'
from(
select IMG.PORT_CODE as '港コード',
PORT_NAME as '港名',
SUM(CASE WHEN KIND_CODE = 110 THEN AMT ELSE 0 END) as '入国者数',
SUM(CASE WHEN KIND_CODE = 120 THEN AMT ELSE 0 END) as '出国者数'
from 
IMMIGRATION IMG
inner join
PORT PT
on
IMG.PORT_CODE = PT.PORT_CODE
inner join
GRP GP
on
IMG.GROUP_CODE = GP.GROUP_CODE
GROUP BY
IMG.PORT_CODE,
PORT_NAME
having
GROUP_NAME = '外国人')
where 
入国者数>出国者数
order by 
差分 desc