sql subtract
I think this is what you're looking for. NEW_BAL is the sum of QTYs subtracted from the balance:
SELECT master_table.ORDERNO,
master_table.ITEM,
SUM(master_table.QTY),
stock_bal.BAL_QTY,
(stock_bal.BAL_QTY - SUM(master_table.QTY)) AS NEW_BAL
FROM master_table INNER JOIN
stock_bal ON master_bal.ITEM = stock_bal.ITEM
GROUP BY master_table.ORDERNO,
master_table.ITEM
If you want to update the item balance with the new balance, use the following:
UPDATE stock_bal
SET BAL_QTY = BAL_QTY - (SELECT SUM(QTY)
FROM master_table
GROUP BY master_table.ORDERNO,
master_table.ITEM)
This assumes you posted the subtraction backward; it subtracts the quantities in the order from the balance, which makes the most sense without knowing more about your tables. Just swap those two to change it if I was wrong:
(SUM(master_table.QTY) - stock_bal.BAL_QTY) AS NEW_BAL