SQL Cumulative Sum not showing when group by values are missing

I have some monthly data as below:

Month     | Category          | Monthly Value
2020-07-01| Food              | 1
2020-07-01| Entertainment     | 4
2020-08-01| Entertainment     | 2
2020-09-01| Entertainment     | 1

I want to calculate the cumulative sum for each Category and get the result as below:

Month       |  Category     | Cumulative Sum
2020-07-01  |  Food         |   1
2020-08-01  |  Food         |   1
2020-09-01  |  Food         |   1
2020-07-01  | Entertainment |   4
2020-08-01  | Entertainment |   6
2020-09-01  | Entertainment |   7

I’m writing the window sum query as below:

    SELECT
      month
    , category
    , sum("monthly value") OVER (PARTITION BY "category" ORDER BY "month" ASC ROWS UNBOUNDED PRECEDING) AS "Cumulative Sum"
    from (
select date_trunc('month', daily_date) as month, category, sum(daily_value) as "monthly value"
from sample_table 
group by date_trunc('month', daily_date) as month, category)

But, I’m getting as follows:

Month           |  Category     | Cumulative Sum
    2020-07-01  |  Food         |   1
    2020-07-01  | Entertainment |   4
    2020-08-01  | Entertainment |   6
    2020-09-01  | Entertainment |   7

Why is “Food” Category’s Cumulative Sum not showing up for the months of 2020-08-01 and 2020-09-01? How can I make the result be displayed as expected (shown in the 2nd table).

I’m using Redshift btw. Thanks!

Answer

Use a cross join to generate the rows and then left join to bring in the values:

select m.month, c.category, t.monthly_value,
       sum(t.monthly_value) over (partition by c.category order by m.month) as running_monthly_value
from (select distinct month from t) m cross join
     (select distinct category from t) c left join
     t
     on t.month = m.month and t.category = c.category;