Query to fill empty rows using select statement - oracle

Question

I'm trying to fill the empty Opening Balance cells for the remaining months using January months data. There are missing months as well. I have tried below approach and consumes a lot of time even to complete initial query.
The fact data count is approximately a million.
The method I tried is below,
Creating a date table with only January EOM dates
Cross join with distinct account key from the fact data with the date table created.
Then try to fill the remaining fields using another left join query by building a dynamic unique key using ACC+EOM.
This approach is taking too much time to execute. Any suggestions please
Input
Output required

Answer 1

You can generate the months using a recursive CTE or just a list. Then use a cross join and left join to bring in the data and window functions to fill in missing values that you want filled in:
with dates as (
select eomonth(convert(date, '2020-01-01')) as eom
union all
select eomonth(dateadd(1, month, eom))
from dates
where eom < '2020-12-01'
)
select a.acc_code, dates.eom,
max(t.ob) over (partition by a.acc_code) as ob,
t.trans
from yyyymm cross join
(select distinct acc_code from t) a left join
t
on t.eom = dates.eom and t.acc_code = a.acc_code

Answer 2

This might work for you. First, you need to generate last day rows for every month of the year, then you can make a cross join with the distinct values of acc_code. And finally, you will need to use an analytic function such as last_value with ignore nulls option to get the last known value for every null value in the ob column.
with your_table(acc_code, eom, ob, trans) as (
select 111111, date '2020-01-31', 2000, 2337 from dual union all
select 111111, date '2020-02-29', cast(null as number), 2149 from dual union all
select 111111, date '2020-03-31', cast(null as number), 2602 from dual union all
select 111111, date '2020-04-30', cast(null as number), 2621 from dual union all
select 111111, date '2020-05-31', cast(null as number), 2870 from dual union all
select 111111, date '2020-06-30', cast(null as number), 2652 from dual union all
select 111111, date '2020-09-30', cast(null as number), 2283 from dual union all
select 111111, date '2020-10-31', cast(null as number), 2680 from dual union all
select 111111, date '2020-11-30', cast(null as number), 2928 from dual union all
select 111111, date '2020-12-31', cast(null as number), 2698 from dual union all
--
select 111112, date '2020-01-31', 3490, 3212 from dual union all
select 111112, date '2020-02-29', cast(null as number), 3605 from dual union all
select 111112, date '2020-03-31', cast(null as number), 3953 from dual union all
select 111112, date '2020-05-31', cast(null as number), 3076 from dual union all
select 111112, date '2020-06-30', cast(null as number), 3638 from dual union all
select 111112, date '2020-07-31', cast(null as number), 3271 from dual union all
select 111112, date '2020-09-30', cast(null as number), 3653 from dual union all
select 111112, date '2020-10-31', cast(null as number), 3075 from dual union all
select 111112, date '2020-11-30', cast(null as number), 3258 from dual union all
select 111112, date '2020-12-31', cast(null as number), 3681 from dual
)
, months_last_days (last_dy) as (
select
last_day(
add_months(date '2020-01-01', level-1)
)
from dual
connect by level <= 12
)
, required_rows as (
select x.ACC_CODE, m.last_dy
from months_last_days m
cross join (select distinct ACC_CODE from your_table) x
)
select rr.ACC_CODE acc
, rr.last_dy eom
, last_value(ob ignore nulls)
over(partition by rr.ACC_CODE order by rr.last_dy) ob
, trans
from required_rows rr
left join your_table t
on (rr.ACC_CODE = t.ACC_CODE
and rr.last_dy = t.eom)
order by 1, 2
;