with temp as(
  select 'China' nation,'Guangzhou' city from dual union all
  select 'China' nation,'Shanghai' city from dual union all
  select 'China' nation,'Beijing' city from dual union all
  select 'USA' nation,'New York' city from dual union all
  select 'USA' nation,'Bostom' city from dual union all
  select 'Japan' nation,'Tokyo' city from dual 
)
select nation,listagg(city,',') within GROUP (order by city)
from temp
group by nation

with temp as(
  select 500 population,'China' nation,'Guangzhou' city from dual union all
  select 1500 population,'Shanghai' city from dual union all
  select 500 population,'Beijing' city from dual union all
  select 1000 population,'USA' nation,'New York' city from dual union all
  select 500 population,'Bostom' city from dual union all
  select 500 population,'Japan' nation,'Tokyo' city from dual 
)
select population,nation,city,') within GROUP (order by city) over (partition by nation) rank
from temp