Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (a, b, c, ym) AS
SELECT 1, 2,  1, DATE '2023-01-01' FROM DUAL UNION ALL
SELECT 1, 2,  7, DATE '2024-09-01' FROM DUAL UNION ALL
SELECT 2, 2,  8, DATE '2024-04-01' FROM DUAL;
3 rows affected

SELECT t.a,
       t.b,
       COALESCE(t.c, 0) AS c,
       ADD_MONTHS(t.year, c.month - 1) AS ym
FROM   (
         SELECT LEVEL AS month
         FROM   DUAL
         CONNECT BY LEVEL <= 12
       ) c
       LEFT OUTER JOIN (
         SELECT a, b, c, TRUNC(ym, 'YY') AS year, EXTRACT(MONTH FROM ym) AS month
         FROM   table_name
       )t
       PARTITION BY (t.a, t.b, t.year)
       ON (t.month = c.month)


A
B
C
YM


1
2
1
2023-01-01 00:00:00

1
2
0
2023-02-01 00:00:00

1
2
0
2023-03-01 00:00:00

1
2
0
2023-04-01 00:00:00

1
2
0
2023-05-01 00:00:00

1
2
0
2023-06-01 00:00:00

1
2
0
2023-07-01 00:00:00

1
2
0
2023-08-01 00:00:00

1
2
0
2023-09-01 00:00:00

1
2
0
2023-10-01 00:00:00

1
2
0
2023-11-01 00:00:00

1
2
0
2023-12-01 00:00:00

1
2
0
2024-01-01 00:00:00

1
2
0
2024-02-01 00:00:00

1
2
0
2024-03-01 00:00:00

1
2
0
2024-04-01 00:00:00

1
2
0
2024-05-01 00:00:00

1
2
0
2024-06-01 00:00:00

1
2
0
2024-07-01 00:00:00

1
2
0
2024-08-01 00:00:00

1
2
7
2024-09-01 00:00:00

1
2
0
2024-10-01 00:00:00

1
2
0
2024-11-01 00:00:00

1
2
0
2024-12-01 00:00:00

2
2
0
2024-01-01 00:00:00

2
2
0
2024-02-01 00:00:00

2
2
0
2024-03-01 00:00:00

2
2
8
2024-04-01 00:00:00

2
2
0
2024-05-01 00:00:00

2
2
0
2024-06-01 00:00:00

2
2
0
2024-07-01 00:00:00

2
2
0
2024-08-01 00:00:00

2
2
0
2024-09-01 00:00:00

2
2
0
2024-10-01 00:00:00

2
2
0
2024-11-01 00:00:00

2
2
0
2024-12-01 00:00:00

A	B	C	YM
1	2	1	2023-01-01 00:00:00
1	2	0	2023-02-01 00:00:00
1	2	0	2023-03-01 00:00:00
1	2	0	2023-04-01 00:00:00
1	2	0	2023-05-01 00:00:00
1	2	0	2023-06-01 00:00:00
1	2	0	2023-07-01 00:00:00
1	2	0	2023-08-01 00:00:00
1	2	0	2023-09-01 00:00:00
1	2	0	2023-10-01 00:00:00
1	2	0	2023-11-01 00:00:00
1	2	0	2023-12-01 00:00:00
1	2	0	2024-01-01 00:00:00
1	2	0	2024-02-01 00:00:00
1	2	0	2024-03-01 00:00:00
1	2	0	2024-04-01 00:00:00
1	2	0	2024-05-01 00:00:00
1	2	0	2024-06-01 00:00:00
1	2	0	2024-07-01 00:00:00
1	2	0	2024-08-01 00:00:00
1	2	7	2024-09-01 00:00:00
1	2	0	2024-10-01 00:00:00
1	2	0	2024-11-01 00:00:00
1	2	0	2024-12-01 00:00:00
2	2	0	2024-01-01 00:00:00
2	2	0	2024-02-01 00:00:00
2	2	0	2024-03-01 00:00:00
2	2	8	2024-04-01 00:00:00
2	2	0	2024-05-01 00:00:00
2	2	0	2024-06-01 00:00:00
2	2	0	2024-07-01 00:00:00
2	2	0	2024-08-01 00:00:00
2	2	0	2024-09-01 00:00:00
2	2	0	2024-10-01 00:00:00
2	2	0	2024-11-01 00:00:00
2	2	0	2024-12-01 00:00:00