select 
trunc(next_day(sysdate,1),'mm') 第一周开始,
trunc(next_day(sysdate,1),'dd') 第一周结束,
trunc(next_day(sysdate,1),'dd')+1 第二周开始,
trunc(next_day(sysdate,1)+7) 第二周结束 
from dual;
结果:
第一周开始    第一周结束    第二周开始    第二周结束
2010-05-01    2010-05-02    2010-05-03    2010-05-09

------解决方案--------------------
SQL code
SQL> select week_of_year + 1 - min(week_of_year) over() week_no_of_month,
  2         first_day_of_week,
  3         last_day_of_week
  4    from (
  5          select min(the_day) first_day_of_week,
  6                 max(the_day) last_day_of_week,
  7                 to_char(the_day, 'IW') week_of_year
  8            from (
  9                  --把一个月的日期都列出来
 10                  select start_date + rownum - 1 the_day
 11                    from (
 12                          select to_date('201004' || '01', 'yyyymmdd') start_date,
 13                                 add_months(to_date('201004' || '01', 'yyyymmdd'), 1) - 1 end_date
 14                            from dual
 15                         )
 16                    connect by (start_date + rownum - 1) <= end_date
 17                  ) t
 18            group by to_char(the_day, 'IW')
 19          )
 20  /
 
WEEK_NO_OF_MONTH FIRST_DAY_OF_WEEK LAST_DAY_OF_WEEK
---------------- ----------------- ----------------
               1 2010/4/1          2010/4/4
               2 2010/4/5          2010/4/11
               3 2010/4/12         2010/4/18
               4 2010/4/19         2010/4/25
               5 2010/4/26         2010/4/30

------解决方案--------------------

判断一个月有几周的函数
SQL code
CREATE OR REPLACE FUNCTION weeknumOfMonth(p_month IN VARCHAR2)
RETURN NUMBER
AS
weeknum NUMBER;
begin
SELECT count(DISTINCT to_char(to_date(p_month,'yyyymm')+ROWNUM-1,'iw'))
  into weeknum
FROM dual
CONNECT BY ROWNUM<=add_months(to_date(p_month,'yyyymm'),1)-to_date(p_month,'yyyymm');
RETURN weeknum;
exception
WHEN others THEN
  RETURN -1;
END;

------解决方案--------------------

CREATE OR REPLACE PACKAGE SP_WEEK IS
 -- 功能 : 一个月有几周,开始时间和结束时间
 TYPE CUR IS REF CURSOR;
 PROCEDURE SP_WEEK(P_YEARMONTH   VARCHAR2,
                          C OUT CUR);
END SP_WEEK;

CREATE OR REPLACE PACKAGE BODY SP_WEEK IS
 PROCEDURE SP_WEEK( P_YEARMONTH  VARCHAR2,
                          C OUT CUR) IS
 BEGIN
   IF LENGTH(P_YEARMONTH)=6 THEN
        OPEN C FOR
     SELECT   week_of_year + 1 - min(week_of_year) over() week_no_of_month,
          first_day_of_week,
          last_day_of_week
     from (
           select min(the_day) first_day_of_week,
                  max(the_day) last_day_of_week,
                  to_char(the_day, 'IW') week_of_year
             from (
                  --把一个月的日期都列出来
                  select start_date + rownum - 1 the_day
                    from (
                          select to_date(P_YEARMONTH   || '01', 'yyyymmdd') start_date,