oracle的list函数

LISTAGG(XXX,XXX) WITHIN GROUP( ORDER BY XXX)，

用法就像聚合函数一样，通过Group by语句，把每个Group的一个字段，拼接起来，非常方便
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)  as Cities
from temp  
group by nation