SELECT CASE
           WHEN TO_NUMBER (TO_CHAR (SYSDATE, 'MM')) - 1 = 0 THEN 12
           ELSE TO_NUMBER (TO_CHAR (SYSDATE, 'MM')) - 1
       END
  FROM DUAL;

WITH T AS (
           SELECT  LEVEL MONTH
             FROM  DUAL
             CONNECT BY LEVEL <= 12
          )
SELECT  MONTH,
        MOD(MONTH + 10,12) + 1 PREVIOUS_MONTH
  FROM  T
/

     MONTH PREVIOUS_MONTH
---------- --------------
         1             12
         2              1
         3              2
         4              3
         5              4
         6              5
         7              6
         8              7
         9              8
        10              9
        11             10

     MONTH PREVIOUS_MONTH
---------- --------------
        12             11

12 rows selected.

SQL>