Hi,
You can do this based on your sample
string connection = System.Configuration.ConfigurationManager.ConnectionStrings["aaa2"].ConnectionString;
SqlConnection con = new SqlConnection(connection);
con.Open();
SqlCommand cmd = new SqlCommand("Select td_em_id from pa_tds_int_entry WHERE td_em_id = '" + ddlId.SelectedValue + "' ", con);
SqlDataReader dr = cmd.ExecuteReader();
ddlId.DataSource = dr;
ddlId.DataTextField = "td_em_id";
ddlId.DataBind();
I hope this one can help.