按系统来减12个月应该这样的，具体你要从什么时候开始，你把序号修改一下就行了

select t.年月,t.月份 from (

select 

to_char(ADD_MONTHS (SYSDATE,-rownum),'yyyymm') as 年月,

to_char(ADD_MONTHS (SYSDATE,-rownum),'mm') as 月份,

to_char(ADD_MONTHS (SYSDATE,-rownum),'yyyymm')*1 as 排序

 from dual connect by 12>=rownum

) t 

order by t.排序

直接建一个数据表不更方便

SELECT 

  --当前月显示为倒数第四位置即增加三个月开始递减

 ADD_MONTHS(ADD_MONTHS(TRUNC(SYSDATE,'mm'),3), -LEVEL+1 )   AS 月初,

 LAST_DAY(ADD_MONTHS(ADD_MONTHS(TRUNC(SYSDATE,'mm'),3), -LEVEL+1 ))  AS 月末 , 

 TO_CHAR(ADD_MONTHS(ADD_MONTHS(TRUNC(SYSDATE,'mm'),3), -LEVEL+1 ),'yyyy-mm') AS 年月,

 TO_CHAR(ADD_MONTHS(ADD_MONTHS(TRUNC(SYSDATE,'mm'),3), -LEVEL+1 ),'mm') AS 月份

FROM DUAL 

CONNECT BY LEVEL <= 12 

ORDER BY 1  --排序

select 

TO_CHAR(ADD_MONTHS (SYSDATE,-rownum+3),'yyyymm') as 年月,

TO_NUMBER(TO_CHAR(ADD_MONTHS (SYSDATE,-rownum+3),'mm')) as 月份

 from dual connect by 12>=rownum

order by 年月