Postgres 13

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create table t as
    select '2018-01' as month, null::int as average union all
    select '2018-02' as month, null as average union all
    select '2018-03' as month, 4 as average union all
    select '2018-04' as month, 10 as average union all
    select '2018-05' as month, null as average union all
    select '2018-06' as month, 20 as average union all
    select '2018-07' as month, null

SELECT 7


select t.*,
       coalesce(max(average) over (order by month), 0) as imputed_average
from t;


month
average
imputed_average


2018-01
null
0

2018-02
null
0

2018-03
4
4

2018-04
10
10

2018-05
null
10

2018-06
20
20

2018-07
null
20



SELECT 7


select t.*,
       coalesce(max(average) over (partition by non_null_month), 0) as imputed_average
from (select t.*,
             max(month) filter (where average is not null) over (order by month) as non_null_month
      from t
     ) t
 order by month


month
average
non_null_month
imputed_average


2018-01
null
null
0

2018-02
null
null
0

2018-03
4
2018-03
4

2018-04
10
2018-04
10

2018-05
null
2018-04
10

2018-06
20
2018-06
20

2018-07
null
2018-06
20



SELECT 7


select t.*,
       coalesce((array_remove(array_agg(average) over (order by month), null))[cardinality(array_remove(array_agg(average) over (order by month), null))], 0) as imputed_average
from t


month
average
imputed_average


2018-01
null
0

2018-02
null
0

2018-03
4
4

2018-04
10
10

2018-05
null
10

2018-06
20
20

2018-07
null
20



SELECT 7

month	average	imputed_average
2018-01	null	0
2018-02	null	0
2018-03	4	4
2018-04	10	10
2018-05	null	10
2018-06	20	20
2018-07	null	20