select p.id, wmsys.wm_concat(d.day) from plan p
left join plan_day d on p.plan_id=d.plan_id
group by p.plan_id

?

From：

http://cache.baidu.com/c?m=9d78d513d9d437ad4f9b96697c15c0101b4381132ba1d5020ca78438e3732b31506692e66771415a84d13b275fa0131aacb22173441e3df2de8d9f4aaae7c9747ad57a23706bda1654ce43e9941838c1229250f4f544a2adf746d5f3c5d4ab020edd54713bc6b0d0065503cd1ba34164a3f1cc1b530807bd996d39e85e0374df7011e810eee0453441dea8&p=8b2a9237c09b11a053ba872b5f&user=baidu&fm=sc&query=%D0%D0%C1%D0%D7%AA%BB%BB++oracle&qid=8f3c3acb1b4bda78&p1=9