Trying to figure out the sum of a certain variable not total sum using sql
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I'm having a mindblank here but my question is quick. 

I have a table like the following: <br>
Cust    Part    Qty    Rtn Qty <br>
Joe      A1      2        0 <br>
Joe      A2      3        1 <br>
Joe      A1      2        2<br>
Bob      A1      3        4<br>
Bob      A2      4        0

I want the results to be like:

Joe      A1      4        2<br>
Joe      A2      3        1<br>
Bob      A1      3        0<br>
Bob      A2      4        0

I'm trying to figure out how to create a sum for each customer individually according to the part? 