unit gridcalc;

interface

uses

  Windows, Messages, SysUtils, Classes, Graphics, Controls,
  Forms, Dialogs, Grids, DBGrids, ExtCtrls, DBCtrls, DB,
  DBTables, StdCtrls;

type

  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
    procedure TaxAmountCalc(DataSet: TDataset);
  private
    TaxAmount: TFloatField;
  end;

var

  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.TaxAmountCalc(DataSet: TDataset);
begin

  Dataset['TaxAmount'] := Dataset['ItemsTotal'] *
    (Dataset['TaxRate'] / 100);
end;

procedure TForm1.FormCreate(Sender: TObject);
var

  MyTable: TTable;
  MyDataSource: TDataSource;
  MyGrid: TDBGrid;
begin

  MyTable := TTable.Create(Self);

  with MyTable do
  begin
    DatabaseName := 'DBDemos';
    TableName := 'Test.DB';
    OnCalcFields := TaxAmountCalc;

    with FieldDefs do
    begin
      Add('ItemsTotal', ftCurrency, 0, false);
      FieldDefs[0].CreateField(MyTable);
      Add('TaxRate', ftFloat, 0, false);
      FieldDefs[1].CreateField(MyTable);
      TFloatField(Fields[1]).DisplayFormat := '##.0%';
      TaxAmount := TFloatField.Create(MyTable);

      with TaxAmount do
      begin
        FieldName := 'TaxAmount';
        Calculated := True;
        Currency := True;
        DataSet := MyTable;
        Name := MyTable.Name + FieldName;
        MyTable.FieldDefs.Add(Name, ftFloat, 0, false);
      end;
    end;
    MyTable.CreateTable;
  end;

  MyDataSource := TDataSource.Create(Self);
  MyDataSource.DataSet := MyTable;
  MyGrid := TDBGrid.Create(Self);

  with MyGrid do
  begin
    Parent := Self;
    Align := alClient;
    DataSource := MyDataSource;
  end;

  MyTable.Active := True;
  Caption := 'Новая таблица ' + MyTable.TableName;
end;

end.