select
    p.PORT_CODE as 港コード,
    PORT_NAME as 港名,
    im.AMT as入国者数,
    im2.AMT as 出国者数,
    (im.AMT - im2.AMT) as 差分
from PORT p
left join 
    (select PORT_CODE, AMT from IMMIGRATION where KIND_CODE = '110' and GROUP_CODE = '120') as im
        on (p.PORT_CODE = im.PORT_CODE)
left join 
    (select PORT_CODE, AMT from IMMIGRATION where KIND_CODE = '120' and GROUP_CODE = '120') as im2
        on (p.PORT_CODE = im2.PORT_CODE)
group by p.PORT_CODE
having 差分 > 0
order by 差分 desc, p.PORT_CODE desc