create table t1(d_date date,d_count number);

insert into t1 values (date'2012-01-01',1000);
insert into t1 values (date'2012-01-02',500);
insert into t1 values (date'2012-01-03',500);
insert into t1 values (date'2012-02-01',800);
insert into t1 values (date'2012-02-02',200);
insert into t1 values (date'2012-03-01',300);
insert into t1 values (date'2012-04-01',400);
insert into t1 values (date'2012-07-01',700);


select to_char(a.c1,'yyyy-mm') 月份,nvl(sum(t1.d_count),0) 数量
from
(SELECT ADD_MONTHS(DATE'2011-12-01',LEVEL) AS c1
    FROM DUAL
   CONNECT BY LEVEL <= 12) a left join t1 on to_char(a.c1,'yyyy-mm')=to_char(t1.d_date,'yyyy-mm')
group by to_char(a.c1,'yyyy-mm')
order by to_char(a.c1,'yyyy-mm')

       月份     数量
-----------------------------------------
1    2012-01    2000
2    2012-02    1000
3    2012-03    300
4    2012-04    400
5    2012-05    0
6    2012-06    0
7    2012-07    700
8    2012-08    0
9    2012-09    0
10    2012-10    0
11    2012-11    0
12    2012-12    0

------解决方案--------------------
SQL code

--构建1年的12个月
SELECT ADD_MONTHS(DATE'2011-12-01',LEVEL) AS c1
    FROM DUAL
   CONNECT BY LEVEL <= 12

--构建一个月的日期
select to_char(to_date('2012-03', 'YYYY-MM') + rownum - 1, 'YYYY-MM-DD') as sdate from dual
                      connect by rownum <= to_number(to_char(last_day(to_date('2012-03', 'YYYY-MM')), 'dd'))