group by to_char(to_date(year || month, 'yyyymm'), 'q')
 
sql>select year, month, to_char(to_date(year || month, 'yyyymm'), 'q'), amount
  2   from t;
 
     YEAR MO T    AMOUNT
--------- -- - ---------
     1998 01 1         2
     1998 02 1         4
     1998 03 1         6
     1998 04 2         8
     1998 05 2        10
     1998 06 2        12
     1998 07 3        14
     1998 08 3        16
     1998 09 3        18
     1998 10 4        20
     1998 11 4        22
     1998 12 4        24
 
12 rows selected.
 
sql>select to_char(to_date(year || month, 'yyyymm'), 'q') quarter, sum(amount)
  2    from t
  3   group by to_char(to_date(year || month, 'yyyymm'), 'q');
 
Q SUM(AMOUNT)
- -----------
1          12
2          30
3          48
4          66