public static DataTable Select(string sql)
        {
            using (DbConnection conn = factory.CreateConnection())
            {
                try
                {
                    conn.ConnectionString = connectionString;
                    conn.Open();

                    DbDataAdapter adapter = factory.CreateDataAdapter();
                    adapter.SelectCommand = conn.CreateCommand();
                    adapter.SelectCommand.CommandText = sql;

                    DataTable table = new DataTable();
                    adapter.Fill(table);
                    return table;
               }
                catch
                {
                    conn.Close();
                    return null;
                }
            }
        }