在oracle中如何取得指定日期是该年的第几周

select to_char(to_date('2009-11-9','YYYY-MM-DD'),'iw') from dual

select to_char(sysdate,'iw') from dual;

select next_day(to_date('20100101', 'yyyymmdd'), 7), next_day(trunc(sysdate,'year'), 7) from dual

我这个语句有问题，lllmouse写得对