Trigger updates more values

Question

Hi guys, I have the trigger below on my db, the trigger is suppose to update a column "Total_qty" in tbl_supply_s table with a value "qty_sold" from vw_receipt_pos. but depending on the number of record that was entered from the application, the triger doubles the update. take for instance, if someone enters two rows with qty_sold = 2 for the two rows, the triger updates total_qty to minus 8 instead of minus 4.
Pls guys help me look at the triger n tell me the problem. thanks
CREATE OR REPLACE TRIGGER "TRG_UPDATE_TOTAL_QTY"
AFTER insert on "TBL_SALES_DETAILS"
begin
declare
v_sales_id number;
v_supply_id number;
v_qty number;
CURSOR cur_update_total IS SELECT distinct supply_id, qty_sold FROM VW_RECEIPT_POS where sales_id = (select max(sales_id) from tbl_sales_s);
begin
select max(sales_id) into v_sales_id from tbl_sales_s;
OPEN cur_update_total;
LOOP
FETCH cur_update_total INTO v_supply_id, v_qty;
EXIT WHEN cur_update_total%NOTFOUND;
update tbl_supply_s
set total_qty = total_qty - v_qty
where supply_id = v_supply_id;
END LOOP;
CLOSE cur_update_total;
end;
end;

Answer 1

Hi,
If I insert 5 rows in one transaction then your trigger fires ONCE and the updates will not be record specific.
For this trigger to work you need to coordinate it behaviour with the rows that have been updated, to do this people typically create a before trigger to clear records in a package, a row level trigger to record which records have been updated, and a table level trigger, not unlike your own, that uses the records that have been updated to correlate their cursor updates. This can be simplified if you write a trigger against a view (to avoid mutating table issues), but for a full description look for 'Mutating Table' on 'Ask Tom' or even this site...
regards,
Robert.