select 
PORT.PORT_CODE AS 港コード,
PORT_NAME AS 港名,
sum(a.amt) as 入国者数,
sum(b.amt) as 出国者数,
sum(a.amt) - sum(b.amt) as 差分
from PORT 
inner join immigration a on a.PORT_CODE = PORT.PORT_CODE and a.GROUP_CODE = '120' and a.KIND_CODE = '110'
inner join immigration b on b.PORT_CODE = PORT.PORT_CODE and b.GROUP_CODE = '120' and b.KIND_CODE = '120'
group by PORT.PORT_CODE
having 差分 > 0
order by 差分 desc;