Decrement a value in a database.. via string grid... on an onclick.. in delphi!

Hi there

For a bit of background, my project is a sort of EPOS/Stock management system.

Everything is now all good, except I haven't been able to figure out how to decrement the stock values as they go through the transaction process.

There are two tables in the database - Product and Takings.. All the stock is stored in Product, obviously. I need to decrement the quantityinstock field when items are sold, of course.

It works by displaying all stock in a dbgrid. There is a query component which allows the user to find an item of stock. The user selects the record in question, then an onclick adds it to a strgrid, which keeps track of items in the current transaction. The code for this onclick is:

procedure TFrmEPOS.BtnEPOSAddClick(Sender: TObject);
begin
  BtnEPOSAdd.Enabled := false;
  { Write current record fieldvalues to string grid }
  StrGridTransaction.Cells[0,Strrow]:=EPOSQuery.FieldValues['ProductName'];
  StrGridTransaction.Cells[1,Strrow]:=EPOSQuery.FieldValues['ProductCost'];
  { Strrow points to next row to modify - is incremented as additions made }
  Strrow := Strrow + 1;
  { If statement disables user's ability to add to grid if full - delete button
  is re-enabled. }
  if Strrow = 17 then
    begin
      BtnEPOSAdd.Enabled := false;
    end;
  BtnEPOSdelete.Enabled := true;
end;


When all items desired are added to the strgrid, the user clicks another button to total the cost, then a final button to register cash tender and calculate change. This final onclick also posts the transaction date and takings to the takings table. The code for that is here:

procedure TFrmEPOS.BtnTenderClick(Sender: TObject);
var
  tender, change : real;
  count: integer;
begin
  tender := StrToFloat(inputbox('Tender','Please enter cash tendered.', ''));
  if tender >= total then
    { allows user to enter cash tendered and calculates change }
    begin
      change := Tender - total;
      LblEPOSChange.Caption := '£'+FloatToStr(change);
      { appending takings table }
      FrmTakings.TakingsQuery.Close;
      FrmTakings.ADOTakings.Append;
      FrmTakings.ADOTakings.Fieldvalues['TransactionDate']:=date;
      FrmTakings.ADOTakings.FieldValues['TransactionTakings']:=total;
      FrmTakings.ADOTakings.Post;
      FrmTakings.TakingsQuery.Open;
      { Clears the string grid of all values }
      Strrow := Strrow +1;
      for count := 0 to Strrow do
      begin
        StrGridTransaction.Cells[1,Count]:='';
        StrGridTransaction.Cells[0,Count]:='';
      end;
        { re enables transaction controls for new transaction }
        MmoTransactionTotal.Text:='';
        BtnEposAdd.Enabled := true;
        BtnEposDelete.Enabled := true;
        BtnEposQuantity.Enabled := true;
        BtnEposTotal.Enabled := true;
        Strrow := 0;
    end
  else
    begin
      ShowMessage('Tender cannot be less than transaction cost!');
    end;
end;


At this point don't mind if the solution is quite clunky, just that it works!

Cheers in advance!!