一点点 WHERE 以及 GROUP BY :

SQL> with test (datum, value) as
  2  -- sample data
  3    (select timestamp '2020-05-10 00:00:00.000',  3 from dual union all
  4     select timestamp '2020-05-20 00:00:00.000',  7 from dual union all
  5     select timestamp '2020-04-08 00:00:00.000', 12 from dual union all
  6     select timestamp '2020-04-10 00:00:00.000', 28 from dual union all
  7     select timestamp '2020-03-15 00:00:00.000', 13 from dual union all
  8     select timestamp '2020-02-08 00:00:00.000', 12 from dual union all
  9     select timestamp '2020-01-10 00:00:00.000', 28 from dual union all
 10     select timestamp '2019-12-15 00:00:00.000', 13 from dual union all
 11     select timestamp '2019-12-21 00:00:00.000', 18 from dual union all
 12     select timestamp '2019-11-12 00:00:00.000', 17 from dual
 13    )
 14  -- query you need
 15  select to_char(datum, 'yyyy/mm') month,
 16         sum(value) total
 17  from test
 18  where datum >= add_months(trunc(sysdate), -6)
 19  group by to_char(datum, 'yyyy/mm')
 20  order by month desc;

MONTH        TOTAL
------- ----------
2020/05         10
2020/04         40
2020/03         13
2020/02         12
2020/01         28
2019/12         31

6 rows selected.

SQL>

请尝试以下操作

select
    to_char(date, 'MM-YYYY') as months,
    sum(value) as total 
from myTable
where date > SYSDATE - INTERVAL '6' MONTH
group by
    to_char(date, 'MM-YYYY')