ASP.NET 4.5中MySQL的ExecuteNonQuery()异常及查询无结果问题

我基于ASP.NET Framework 4.5开发项目，使用MySQL数据库处理事务，编写了一个通用方法用于处理INSERT、UPDATE等DML操作及SELECT查询，最初采用ExecuteNonQuery()方法，但发现SELECT查询有时无法返回结果。

public DataSet test(string sql, string Connectionstring)
{
    DataSet ds = new DataSet();
    try
    {
        string timeZone = '-05:00'
        using (MySqlConnection conn = new MySqlConnection(Connectionstring))
        {
            try
            {
                conn.Open();
                MySqlCommand cmd;
                if (!string.IsNullOrEmpty(timeZone))
                {
                    cmd = new MySqlCommand();
                    string zoneQuery = "SET SESSION time_zone = '" + timeZone + "';";
                    cmd.CommandText = zoneQuery;
                    cmd.Connection = conn;
                    cmd.CommandTimeout = connectiontimeout;
                    cmd.ExecuteNonQuery();
                    cmd.Dispose();
                }
                cmd = new MySqlCommand();
                cmd.CommandText = sql;
                cmd.Connection = conn;
                cmd.CommandTimeout = connectiontimeout;
                cmd.ExecuteNonQuery();
                MySqlDataAdapter ad = new MySqlDataAdapter(cmd);
                ad.Fill(ds);
                ad.Dispose();
                cmd.Dispose();
                conn.Close();
            }
            finally
            {
                if (conn != null && conn.State != ConnectionState.Closed)
                {
                    conn.Close();
                }
            }
        }
    }
    catch (Exception ex)
    {
     objlogger.WritetoError("Error in test: " + ex.StackTrace.ToString());

    }
    return ds;
}

select distinct(s.st),s.a,s.a1,s.a2,s.a3,s.a4,q.a5,l.a6,t.a7
 from test1 as s
   join test2 as q on s.st = q.st 
   join test3 as t on t.Uid = s.Uid 
   join  test4 as l on q.lt1 = l.lt1 
 where s.is1 = false 
   &&  l.lt1 =8 && q.is_deleted = false && t.Is_Deleted = false &&
   s.Is_Deleted = false && l.Is_Deleted = 0;

Server=localhost;Database=abcdtest;Uid=root;Pwd=test;Connection Timeout=60000;CharSet=utf8;Convert Zero Datetime=true;

public DataSet test(string sql, string Connectionstring)
{
    DataSet ds = new DataSet();
    try
    {
        string timeZone = '-04:00';
        int connectiontimeout = 60000;               
        using (MySqlConnection conn = new MySqlConnection(Connectionstring))
        {
            try
            {
                conn.Open();
                MySqlCommand cmd;
                MySqlDataReader myReader;
                if (!string.IsNullOrEmpty(timeZone))
                {
                    cmd = new MySqlCommand();
                    string zoneQuery = "SET SESSION time_zone = '" + timeZone + "';";
                    cmd.CommandText = zoneQuery;
                    cmd.Connection = conn;
                    cmd.CommandTimeout = connectiontimeout;
                    myReader = cmd.ExecuteReader();
                    myReader.Close();
                    cmd.Dispose();
                }
                MySqlDataAdapter adapter = new MySqlDataAdapter();
                adapter.SelectCommand = new MySqlCommand(sql, conn);
                adapter.Fill(ds);
                conn.Close();
            }
            finally
            {
                if (conn != null && conn.State != ConnectionState.Closed)
                {
                    conn.Close();
                }
            }
        }
    }
    catch (Exception ex)
    {
        objlogger.WritetoError("Error in test : " + ex.Message);
    }
    return ds;
}

内容的提问来源于stack exchange，提问作者santosh