--原表
with tb1(ID ,start_month ,months) as
(select 1 ,'201201' ,2 from dual union all
select 2 ,'201205' ,1 from dual union all
select 3 ,'201203' ,12 from dual)
--原表基础上多构造了一个end_month
,tb2(ID ,start_month,end_month ,months) as
(select ID, start_month,to_char(add_months(to_date(start_month,'yyyymm'),months-1),'yyyymm'),months from tb1)
--构造了一个最长月的时间段201201-201302（14个月）
,tb3 ( month_DETAIL ) as 
(select to_char(add_months(to_date((select min(start_month) from tb2),'yyyymm'),rownum-1),'yyyymm') from dual 
connect by rownum<=(select months_between(to_date(max(end_month),'yyyymm'),to_date(min(start_month),'yyyymm')) from tb2)+1)
--tb2,tb3做连接，连接条件是month_DETAIL的值在start_month,end_month之间（包含）
select ID,start_month,months, month_DETAIL from tb2,tb3 where month_DETAIL between start_month and end_month order by id

------解决方案--------------------

SQL code

SELECT t1.tid,
             t1.start_month,
             t1.months,
             TO_CHAR(ADD_MONTHS(TO_DATE(t1.start_month,'yyyymm'),t2.rn - 1),'yyyymm') month_detail
  FROM tb t1,
       (SELECT ROWNUM rn,
                     max_mon
          FROM (SELECT MAX(months) max_mon
                  FROM tb) m
        CONNECT BY ROWNUM <= max_mon) t2
 WHERE t1.months >= t2.rn
ORDER BY t1.tid,month_detail

------解决方案--------------------
这个是正确思路，我再给你写一个
SQL code


SELECT T1.TID,
       T1.START_MONTH,
       T1.MONTHS,
       TO_CHAR(ADD_MONTHS(TO_DATE(T1.START_MONTH, 'yyyymm'), L), 'yyyymm') MONTH_DETAIL
  FROM 原表 T1,
       (SELECT LEVEL L
          FROM DUAL
        CONNECT BY LEVEL <= (SELECT MAX(MONTHS) MAX_MON FROM 原表)) T2
 WHERE T1.MONTHS >= T2.L
 ORDER BY T1.TID, MONTH_DETAIL