restar en el ejemplo de código de consulta sql

Ejemplo 1: como restar del valor de una tabla en sql

UPDATE Inventory
   SET Quantity = Quantity - 1
 WHERE InventoryID = 2

Ejemplo 2: resta de sql

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

Ejemplo 3: subconsulta en sql

SUBQUERY IS NESTED QUERY INSIDE A 
SELECT, INSERT OR UPDATE METHODS.
OR INSIDE ANOTHER SUBQUERY

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