cnn.Open();
                MySqlCommand cmd = new MySqlCommand();
                cmd.Connection = cnn;
                cmd.CommandText = "select df.* from    dfhistory df,    ( select @test := @test+1 as rank, id, mod(@test,@step) as m from dfhistory ) tx where     tx.id=df.id     AND tx.m=0     and STR_TO_DATE(df.instr_date,'%c/%d/%Y %T') <= SYSDATE() - interval @time hour     and STR_TO_DATE(df.instr_date,'%c/%d/%Y %T') >= SYSDATE() - interval @time hour - interval @interv hour";
                cmd.Parameters.AddWithValue("@test", 0);
                cmd.Parameters.AddWithValue("@step", Convert.ToInt32(step));
                cmd.Parameters.AddWithValue("@time", Convert.ToInt32(time));
                cmd.Parameters.AddWithValue("@interv", Convert.ToInt32(interval));
                cmd.Parameters.AddWithValue("@name", name);
                MySqlDataReader reader = cmd.ExecuteReader();
                while (reader.Read())
                {
                    WidgetInfo wInfo = new WidgetInfo();
                    wInfo.name = reader.GetString(1);
                    wInfo.bid = reader.GetString(3);
                    wInfo.ask = reader.GetString(4);
                    wInfo.date = reader.GetString(5);
                    lst.Add(wInfo);
                }
                cnn.Close();

set @test = 0;

select
      df.*
from
    dfhistory df,
    ( select @test := @test+1 as rank, id, mod(@test,5) as m from dfhistory ) tx
where
     tx.id=df.id
     AND tx.m=0
    and STR_TO_DATE(df.instr_date,'%c/%d/%Y %T') <= SYSDATE() - interval 72 hour
    and STR_TO_DATE(df.instr_date,'%c/%d/%Y %T') >= SYSDATE() - interval 72 hour - interval 24 hour
    and df.instr_name = @name

select @test := @test+1 as rank, id, mod(@test,5) as m from dfhistory 

tx.m=0