请大家帮忙看看是什么错误,谢谢了!

Question

运行时出现:"can't perform this operation on an open dataset" 请大家看看,是哪有问题.谢谢!
 procedure TForm1.ComboBox2Change(Sender: TObject);
var
gradepartment :string;
sql:string;
iteminde:integer;
begin
  iteminde:=combobox2.ItemIndex+1;
  if(iteminde<=9) then
      begin
     gradepartment:=combobox1.Text+'0'+inttostr(iteminde);
      end
  else
      begin
      gradepartment:=combobox1.Text+inttostr(iteminde);
      end;
 sql:='select * from classdata where grade='''+gradepartment+''''
 query1.Close;
 query1.SQL.Clear;
 query1.SQL.Add(sql);
 query1.ExecSQL;
 query1.Open;
 query1.UnPrepare; query1.Close;
 query1.SQL.Clear;
 query1.SQL.Add(sql);
 query1.ExecSQL;
 query1.Open;
 query1.UnPrepare;
end;end.

Answer 1

应为'select * from classdata where grade='+''''+gradepartment+''''

Answer 2

query1.Close;
 query1.SQL.Clear;
 query1.SQL.Add(sql);
 query1.Prepare;
 query1.Open;
 query1.UnPrepare;
  query1.Open;  // 查询用 Open
 query1.ExecSQL;// Delete,Insert,Update 用 ExecSQL