-- Query below shows splitting month number into quarters
-- using Oracle functions DECODE and LEAST.
--  1 to  3 as Quarter 1
--  4 to  6 as Quarter 2
--  7 to  9 as Quarter 3  and
-- 10 to 12 as Quarter 4

SELECT q_tab.v_month month_no,
 DECODE(q_tab.v_val,0, 4,
  DECODE(LEAST(q_tab.v_val,9), q_tab.v_val,
   DECODE(LEAST(q_tab.v_val,6), q_tab.v_val,
     DECODE(LEAST(q_tab.v_val,3),
       q_tab.v_val,1,2),3),4))  Quarter
FROM
  (SELECT LEVEL v_month, MOD(LEVEL,12) v_val 
   FROM DUAL
   CONNECT BY LEVEL <= &i_months) q_tab
;