我需要选择PolygonID列的最大ID。我这样保存我的数据

string sql = "create table Polygons (PolygonId int, PointId int, X double, Y double)";

           // Выполнение нашей команды
           using (SQLiteCommand command = new SQLiteCommand(sql, m_dbConnection))
                {
                    command.ExecuteNonQuery();
                }

                int pointId = 1;
                for (int i = 0; i < listOfCustomPolygons.Count; i++)
                    for (int j = 0; j < listOfCustomPolygons[i].listOfVertexes.Count; j++)
                    {
                        string strSQL =
                            string.Format("INSERT INTO Polygons (PolygonId,PointId,X,Y) Values ('{0}','{1}','{2}','{3}')",
                                            i+1,pointId,listOfCustomPolygons[i].listOfVertexes[j].X,
                                          listOfCustomPolygons[i].listOfVertexes[j].Y );
                        pointId++;

                        using (SQLiteCommand insertCommand = new SQLiteCommand(strSQL, m_dbConnection))
                        {
                            insertCommand.ExecuteNonQuery();
                        }
                    }

之后，我想从表Polygons和列PolygonID中选择最大值，但我得到了一个IndexOutOfRangeException异常。如何才能解决这个问题？

using (SQLiteConnection connection = new SQLiteConnection("Data Source=" + openFileDialog.FileName + ";Version=3;"))
                {
                    connection.Open();
                    string selectMaxId = "Select Max(PolygonId) From Polygons";

                    string selectQuery = "Select * From Polygons";

                    SQLiteCommand selectMaxCmd = new SQLiteCommand(selectMaxId,connection);
                    SQLiteDataReader dataReader = selectMaxCmd.ExecuteReader();
                    int maxId = Convert.ToInt32(dataReader["Select Max(PolygonId) From Polygons"]); // This is don't work! Why?