I am trying to generate a data set for client churn by month and I'm running into an issue. I need to be able to calculate total clients (running total) up to a specific month and total lost clients that month.

The final data should look something like this. You can imagine there were 5 new clients in November, 10 new clients in December and 25 new clients in January.

I run into an issue when trying to calculate running total number of clients. I can't seem to find a way to use rank & window or group by to be able to calculate the total number of clients before a given date. I can easily see how many clients were added within a given month but I cannot seem to achieve a running total.

How might I accomplish this?