select 
港コード,
港名,
sum(入国者) as 入国者数,
sum(出国者) as 出国者数,
sum(入国者)-sum(出国者) as 差分
from
(
select
p.port_code as 港コード,
p.port_name as 港名,
case i.kind_code
    when 110 then i.amt 
    else 0 end as 入国者,
case i.kind_code
    when 120 then i.amt
    else 0 end as 出国者
from
port p,
immigration i,
grp g
where
p.port_code = i.port_code
and
i.group_code = g.group_code
)
group by 港コード,港名
having 差分 > 0
order by 差分 desc, 港コード desc