Problem with gaps in data with generate_series - postgresql

Question

I need a query that returns the cumulative sum of all paid bills per day in the current month.
I've tried a few codes, including this one:
SELECT DISTINCT
month.day,
sum(bills.value) OVER (ORDER BY month.day)
FROM generate_series(1,31) month(day)
LEFT JOIN bills ON date_part('day',bills.payment_date) = month.day
WHERE
(
(date_part('year',bills.payment_date)=date_part('year',CURRENT_DATE)) AND
(date_part('month',bills.payment_date)=date_part('month',CURRENT_DATE))
)
GROUP BY month.day, bills.value, bills.payment_date
ORDER BY month.day
I'm getting:
day | value
1 | 1000
4 | 3000
5 | 5000
The sum is correct, but I'm not getting all the 31 days from the generate_series function. Also, when I remove the DISTINCT command, the query just repeat the days, like:
day | value
1 | 1000
4 | 3000
4 | 3000
4 | 3000
4 | 3000
5 | 5000
5 | 5000
What I want is:
day | value
1 | 1000
2 | 1000
3 | 1000
4 | 3000
5 | 5000
6 | 5000
... | 5000
31 | 5000
Any ideas?

Answer 1

Remove bills.value, bills.payment_date from the group by, then you also don't need the distinct any more. You can also simplify the WHERE clause. But you need to move that condition into the JOIN condition, otherwise it will turn the outer join back into an inner join.
SELECT month.day,
sum(sum(bills.value)) over (order by month.day) as total_value
FROM generate_series(1,31) month(day)
LEFT JOIN bills
ON date_part('day',bills.payment_date) = month.day
AND to_char(bills.payment_date, 'yyyymm') = to_char(current_date, 'yyyymm')
GROUP BY month.day
ORDER BY month.day