I am trying to find the running sum of quantities across accounts over time. Ideally I want to sum the quantities of the current row and the most recent row for each account.

Given the following as example input data

The output I desire would be:

I have tried using a window function like:

select ... sum(quantity) over (partition by account_id order ... rows between unbounded preceding and current row) ...

However this provides only a running total of quantity on each by it's account id.

I have also tried using first_value/lag but again these each only associate with the current row's account id. I'm guessing this is not easily interpreted as a window function but I'm looking for any approach available.

If I use a sum on a row_number() based window, I receive row_number cannot be used here