procedure TForm1.TotalPopulation: double;
var
  Tbl: TTable;
begin
  Result := 0;
  Tbl := TTable.Create(nil);
  try
    tbl.DatabaseName := 'DBDEMOS';
    tbl.TableName := 'COUNTRY';
    Tbl.Open;
    tbl.First;
    while not Tbl.EOF do
    begin
      Result := Result + Tbl.FieldByName('Population').AsFloat;
      Tbl.Next;
    end;
    Tbl.Close;
  finally
    Tbl.Free;
  end;
end;