어떻게 조회하는지에 따라 대응 방법이 달라질 수 있습니다.
사용 쿼리를 보여주세요.

2020년만 조회, 1/2/4/5/7 월에는 값이 들어 있다는 가정하에서 생각해보았습니다.

더 효율적인 방법은 다른분이 올려주실거라 믿으며..

create table test_val (target_year integer, target_month integer, target_val integer);
insert into test_val values (2020, 1, 1000);
insert into test_val values (2020, 2, 2000);
insert into test_val values (2020, 4, 4000);
insert into test_val values (2020, 5, 5000);
insert into test_val values (2020, 7, 7000);
select * from test_val;

with a as (
 select 2020 as target_year, 1 as target_month, 0 as target_val
 union
 select 2020 as target_year, 2 as target_month, 0 as target_val
 union
 select 2020 as target_year, 3 as target_month, 0 as target_val
 union
 select 2020 as target_year, 4 as target_month, 0 as target_val
 union
 select 2020 as target_year, 5 as target_month, 0 as target_val
 union
 select 2020 as target_year, 6 as target_month, 0 as target_val
 union
 select 2020 as target_year, 7 as target_month, 0 as target_val
 union
 select 2020 as target_year, 8 as target_month, 0 as target_val
 union
 select 2020 as target_year, 9 as target_month, 0 as target_val
 union
 select 2020 as target_year, 10 as target_month, 0 as target_val
 union
 select 2020 as target_year, 11 as target_month, 0 as target_val
 union
 select 2020 as target_year, 12 as target_month, 0 as target_val
), b as (
 select test_val.*
 from test_val, a
 where test_val.target_month = a.target_month
), c as (
 select * 
 from a
 where NOT EXISTS (select * from b where a.target_month = b.target_month)
)
select target_year, target_month, target_val
from
(
select * from b
union
select * from c
) as t
order by target_month