Cumulative sum of previous rows for each partition

I want to calculate the cumulative sum of monthly orders for each customer in my database.

For example, I have this data:

And the result should be the same as below:

I used lag() and in the next step, sum() over () but my result was false!

How can I solve this problem?

>Solution :

This should do the trick (DB FIDDLE)

SELECT customer,
       year,
       month,
       cumulative_no_orders = ISNULL(SUM(SUM(no_orders))
                                       OVER (
                                         PARTITION BY customer
                                         ORDER BY year, month 
                                         ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING)
                                     , 0)
FROM   YourTable
GROUP  BY customer,
          year,
          month