postgresql - cumul. sum active customers by month (removing churn) - sql

Question

I want to create a query to get the cumulative sum by month of our active customers. The tricky thing here is that (unfortunately) some customers churn and so I need to remove them from the cumulative sum on the month they leave us.
Here is a sample of my customers table :
customer_id | begin_date | end_date
-----------------------------------------
1 | 15/09/2017 |
2 | 15/09/2017 |
3 | 19/09/2017 |
4 | 23/09/2017 |
5 | 27/09/2017 |
6 | 28/09/2017 | 15/10/2017
7 | 29/09/2017 | 16/10/2017
8 | 04/10/2017 |
9 | 04/10/2017 |
10 | 05/10/2017 |
11 | 07/10/2017 |
12 | 09/10/2017 |
13 | 11/10/2017 |
14 | 12/10/2017 |
15 | 14/10/2017 |
Here is what I am looking to achieve :
month | active customers
-----------------------------------------
2017-09 | 7
2017-10 | 6
I've managed to achieve it with the following query ... However, I'd like to know if there are a better way.
select
"begin_date" as "date",
sum((new_customers.new_customers-COALESCE(churn_customers.churn_customers,0))) OVER (ORDER BY new_customers."begin_date") as active_customers
FROM (
select
date_trunc('month',begin_date)::date as "begin_date",
count(id) as new_customers
from customers
group by 1
) as new_customers
LEFT JOIN(
select
date_trunc('month',end_date)::date as "end_date",
count(id) as churn_customers
from customers
where
end_date is not null
group by 1
) as churn_customers on new_customers."begin_date" = churn_customers."end_date"
order by 1
;

Answer 1

You may use a CTE to compute the total end_dates and then subtract it from the counts of start dates by using a left join
SQL Fiddle
Query 1:
WITH edt
AS (
SELECT to_char(end_date, 'yyyy-mm') AS mon
,count(*) AS ct
FROM customers
WHERE end_date IS NOT NULL
GROUP BY to_char(end_date, 'yyyy-mm')
)
SELECT to_char(c.begin_date, 'yyyy-mm') as month
,COUNT(*) - MAX(COALESCE(ct, 0)) AS active_customers
FROM customers c
LEFT JOIN edt ON to_char(c.begin_date, 'yyyy-mm') = edt.mon
GROUP BY to_char(begin_date, 'yyyy-mm')
ORDER BY month;
Results:
| month | active_customers |
|---------|------------------|
| 2017-09 | 7 |
| 2017-10 | 6 |