Can someone please help me with a SELECT?

Can someone help me with a SELECT? I have a SELECT SELECT YEAR, CUSTOMER, PRODUCT, PERIOD, REGION, SALES from my_table order by YEAR, CUSTOMER, PRODUCT, PERIOD, REGION which works fine. Now here is the hard bit, I want to add a "derived" column : ytd_sales ytd_sales corresponds to the sum of the sales, for every combination of YEAR, CUSTOMER, PRODUCT. In other words I want to find the ytd_sales for the current YEAR, CUSTOMER, PRODUCT regardless of the period and region and include it in the returned row. It has to be pure SQL. As an example, suppose I had only 4 rows in the table , the output would look like : YEAR CUSTOMER PRODUCT PERIOD REGION SALES YTD_SALES 97 1 1 11 1 2 10 97 1 1 11 2 3 10 97 1 1 12 1 4 10 97 1 1 12 2 1 10 I hope I've explained it well! Any suggestions gratefully received!