ExecuteNonQuery() is not working with mysql?

Question

I am working on asp.net frame work with 4.5 version and i am using mysql database for db transaction. I am using method which is common for DML operations such as insert,update and for select query also and I am using ExecuteNonQuery() method.

Here are my code sample

    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("\nError in test: " + ex.StackTrace.ToString());

        }
        return ds;
    }

And my sql is:

 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;

In the above code select query is working but sometimes it is not giving result.

Here is my connection string:

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

I also tried with ExecuteReader() please see below

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;
    }