SQL - sum of some rows, minus sum of other rows

Question

I have a table in mySQL with the following columns:

CUSTOMER_CODE | TRANS_TYPE | TRANS_VALUE

TRANS_TYPE could be either \"DRINV\" (a sale) or \

Answer 1

select customer_code,
       sum(case
             when trans_type = 'DRINV' then
              trans_value
             else
              -trans_value
           end) as net_sales
  from dr_trans
 group by customer_code