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