麻烦各位大神帮忙看看C# sql

            string connstg = "server=.;database=Scholarship;integrated security=SSPI";
            SqlConnection conn = new SqlConnection(connstg);
            string Sql="select " + comboBox1.Text + "  from zhuanye where UserNum='" + num + "'";
            conn.Open();
            SqlCommand cmd = new SqlCommand(Sql, conn);
            SqlDataReader sdr =cmd.ExecuteReader();
            sdr.Read();
            if (sdr.HasRows)
                            ｛
                                                MessageBox.Show("添加成功！");
                                                                    textBox1.Text = "";
                            ｝

                            这段代码，查询结果为真，判断的时候结果一直为假