Получить результат запроса

 MySqlDataAdapter adapter2 = new MySqlDataAdapter("select count(case percents.RIGHT_ANSWER when '1' then 1 else null end)/ count(percents.questions)*100 'res' from(  select qu.id_questions questions, count(case au.priznak when '1' then 1 else null end) = count(au.priznak) AND  count(au.priznak) = count(case a.priznak when '1' then 1 else null end) RIGHT_ANSWER  from  question_user qu left join  answer a on qu.id_questions = a.id_questions left join  answer_user au on a.id_answer = au.id_answer where qu.id_themes=@themesID and qu.id_users=userID group by qu.id_questions) percents", conn);
 
            adapter2.SelectCommand.Parameters.AddWithValue("@themesID", themesId);
            adapter2.SelectCommand.Parameters.AddWithValue("@userID", userId);

  String res = null;
            MySqlDataReader datareader = command.ExecuteReader();
              while (datareader.Read())
                {
                   res=datareader["res"]+"";
                }
              datareader.Close();
              MessageBox.Show(res);

var res = command.ExecuteScalar().ToString();
                if (res != null)
                   {
                       Form6 f6 = new Form6();
                       f6.Show();
                       this.Hide();
                   }