Calculating monthly churn - on SQL SERVER - sql

Question

I am trying to calculate a monthly churn rate (for a given month: number_of_people_who_unsubscribed / number_of_subscribers_at_beginnin
enter image description here
That gives me a single percentage of users who unsubscribed during January. However, I'd like to output this percentage for every month, so I can display it as a line chart. I'm not sure where to start - it feels like I need to loop and run the query for each month but that feels wrong. How could I make the same calculation, but without specifying the month manually? We can assume that there is at least one start_date and one end_date per month, so some kind of group by might work.
WITH
date_range AS (
SELECT '2022-10-01' AS start_date, '2022-10-31'AS end_date
),
start_accounts AS
(
SELECT DISTINCT ProductContractId
FROM HD s INNER JOIN date_range d ON
s.FirstInvoiceDate<= d.start_date
AND (s.ItemRejectDate>d.start_date or s.ItemRejectDate is null)
),
end_accounts AS
(
SELECT DISTINCT ProductContractId
FROM HD s INNER JOIN date_range d ON
s.FirstInvoiceDate<= d.end_date
AND (s.ItemRejectDate>d.end_date or s.ItemRejectDate is null)
),
churned_accounts AS
(
SELECT s.ProductContractId
FROM start_accounts s
LEFT OUTER JOIN end_accounts e ON
s.ProductContractId=e.ProductContractId
WHERE e.ProductContractId is null
),
start_count AS (
SELECT COUNT(*) AS n_start FROM start_accounts
),
churn_count AS (
SELECT COUNT(*) AS n_churn FROM churned_accounts
)
SELECT
convert(numeric(10,4),(n_churn* 1.0/n_start ))*100
AS churn_rate,
(1.0-(n_churn/n_start)*100)
AS retention_rate,
n_start,
n_churn
FROM start_count, churn_count
Ultimately I'm looking for an output that looks something like:
enter image description here

Answer 1

I don't have SQL Serve setup yet (working on it) but here is a solution that works on the Postgres schema from the book Fighting Churn With Data (https://www.fightchurnwithdata.com, which is the basis for the original query in the question.) The key is to select multiple dates as a constant table in the first CTE (or use some external table defined for this purpose) and then carry it through all the other CTE's so it calculates multiple churn rates at once. The end date of each churn calculation period is used with a date interval expression (that will definitely have to change for SQLServer but I think the rest of it should work...)
WITH
calc_dates AS (
SELECT generate_series(min(start_date)
, max(start_date)-interval '1month'
, interval '1 month') AS start_date
FROM subscription
),
start_accounts AS
(
SELECT DISTINCT d.start_date, account_id
FROM SUBSCRIPTION s INNER JOIN calc_dates d ON
s.START_DATE<= d.start_date
AND (s.END_DATE>d.start_date or s.END_DATE is null)
),
end_accounts AS
(
SELECT DISTINCT d.start_date, account_id
FROM SUBSCRIPTION s INNER JOIN calc_dates d ON
s.START_DATE<= (d.start_date+interval '1 month')
AND (s.END_DATE>(d.start_date+interval '1 month') or s.end_date is null)
),
churned_accounts AS
(
SELECT s.start_date, s.account_id
FROM start_accounts s
LEFT OUTER JOIN end_accounts e ON
s.account_id=e.account_id
and s.start_date = e.start_date
WHERE e.account_id is null
),
start_count AS (
SELECT start_date, COUNT(*)::FLOAT AS n_start
FROM start_accounts
group by start_date
),
churn_count AS (
SELECT start_date, COUNT(*)::FLOAT AS n_churn
FROM churned_accounts
group by start_date
)
SELECT s.start_date,(s.start_date+interval '1 month')::date as end_date,
(n_churn* 1.0/n_start )*100
AS churn_rate
FROM start_count s
inner join churn_count c
on s.start_date=c.start_date
order by s.start_date;
This gives the following output when run against the book schema:
start_date
end_date
churn_rate
2020-02-01
2020-03-01
6.42486011191047
2020-03-01
2020-04-01
5.63271604938272
2020-04-01
2020-05-01
4.83752524325317
I'm working on getting a SQLServer running and will try to update the solution soon...