-- oracle 依靠 connect by 很容易就做到了
-- 测试数据：
with A as (
  select 1 id , 'aaa' title,'******' content,'sender1' sender from dual union all
  select 2 id , 'bbb' title,'******' content,'sender2' sender from dual union all
  select 3 id , 'ccc' title,'******' content,'sender3' sender from dual
)
, B as  (
  select 1 id , 1 aid, 'R01' Reciver,'********' content from dual union all
  select 2 id , 1 aid, 'R02' Reciver,'********' content from dual union all
  select 3 id , 1 aid, 'R03' Reciver,'********' content from dual union all
  select 4 id , 2 aid, 'R04' Reciver,'********' content from dual union all
  select 5 id , 2 aid, 'R05' Reciver,'********' content from dual 
) 
-- >> start here
, nb as (
  select aid,ltrim(sys_connect_by_path(Reciver,','),',') line
  from 
  ( select row_number() over(partition by aid order by id) rn,id,aid,reciver from B ) b
  where connect_by_isleaf=1
  start with b.rn=1
  connect by prior b.aid=b.aid
  and prior b.rn+1=b.rn
)
select a.id,a.title,a.content,a.sender,nb.line recivers
from A inner join nb
on A.id=nb.aid
-- >> end here
-- >> 结果：
        ID TIT CONTEN SENDER  RECIVERS
---------- --- ------ ------- ------------------------------
         1 aaa ****** sender1 R01,R02,R03
         2 bbb ****** sender2 R04,R05

-- SQL Server 2005 或以上版本可以借助递归CTE：
-- 测试数据：
with A(id,title,content,sender) as (
  select 1 id , 'aaa' title,'******' content,'sender1' sender  union all
  select 2 id , 'bbb' title,'******' content,'sender2' sender  union all
  select 3 id , 'ccc' title,'******' content,'sender3' sender 
)
, B(id,aid,reciver,content) as  (
  select 1 id , 1 aid, 'R01' Reciver,'********' content union all
  select 2 id , 1 aid, 'R02' Reciver,'********' content union all
  select 3 id , 1 aid, 'R03' Reciver,'********' content union all
  select 4 id , 2 aid, 'R04' Reciver,'********' content union all
  select 5 id , 2 aid, 'R05' Reciver,'********' content 
) 
-- >> start here
, B_RN_CN(rn,cn,id,aid,reciver) as (
  select 
    row_number() over(partition by aid order by id) rn,
    COUNT(*) over(partition by aid) cn,
    id,aid,reciver 
  from B
)
, cte as (
  select rn,cn,id,aid,reciver,cast(reciver as varchar) line from B_RN_CN where rn=1
  union all 
  select b.rn,b.cn,b.id,b.aid,b.reciver,cast ( line+','+b.reciver as varchar)
  from cte c,B_RN_CN b where c.aid=b.aid and b.rn=c.rn+1
)
select a.id,a.title,a.content,a.sender,c.line recivers 
from cte c,A
where c.rn=c.cn
and c.aid=a.id
-- end here
-- 结果：
id          title content sender  recivers
----------- ----- ------- ------- ------------------------------
1           aaa   ******  sender1 R01,R02,R03
2           bbb   ******  sender2 R04,R05