I have a table wiht 3 fiels

customer

quantity

% of quantity in the total (sum quantity)

I need a 4th colum that accumulate the percentagem:
how could I have this columm? What should stay the select?

example

customer quant percent acum

customer a 40 61 61%
customer b 20 31% 93%
customer c 5 8% 101%
...