string connString = "server=(local);uid=sa;pwd=;database=Northwind";   
            string cmdText = "select * from ZI ";   
            SqlConnection conn = new SqlConnection(connString);   
            SqlDataAdapter adp = new SqlDataAdapter(cmdText,conn);   
            DataSet ds = new DataSet();   
            adp.Fill(ds);   
                            DropDownList1.DataSource = ds;   
            DropDownList1.DataTextField = "Address_name";   
            DropDownList1.DataValueField = "Relegation_ID";   
            DropDownList1.DataBind();
我想要显示Address_name的字段，不过要符合Relegating_ID为空的条件，应该怎么判断

请高手帮忙下！！！

请高手速来帮忙啊！！！