如何统计一个表里边 某个数据有多少项 用来限制选课人数

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Drawing;
using System.Data;
using System.Data.SqlClient;public partial class _Default : System.Web.UI.Page
{
    //public RadioButton r1;
    protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=mytest;Integrated Security=True");
        con.Open();
        SqlDataAdapter da = new SqlDataAdapter("select * from xuankebiao", con);
        DataSet ds = new DataSet();//数据集
        da.Fill(ds);
        gridview1.DataSource = ds.Tables[0];
        gridview1.DataBind();
        con.Close();    }
    protected void gridview1_SelectedIndexChanged(object sender, EventArgs e)
    {
        //获取当前行的第0列的文本
        string cno = gridview1.SelectedRow.Cells[0].Text;
        string cn = gridview1.SelectedRow.Cells[1].Text;
        //将值赋值给session
        Session["cno"] = cno;
        Session["cn"] = cn;
        //Response.Redirect("huitie.aspx");
        
        GridView gridview2 = new GridView();        
         //r1 = new RadioButton();
        SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=mytest;Integrated Security=True");
        con.Open();
        SqlCommand cmd = new SqlCommand("select * from xuankebiao where cno='"+cno+"' and cname='"+cn+"'",con);
        SqlDataReader sdr = cmd.ExecuteReader();
        while (sdr.Read())
        {
            La1.Text="课程编号："+Server.HtmlEncode(sdr["cno"].ToString())+"        ";
            La2.Text="课程名:"+Server.HtmlEncode(sdr["cname"].ToString())+"         ";
            La3.Text = "任课教师:"+Server.HtmlEncode(sdr["teaname"].ToString())+"           ";
            La4.Text = "总人数："+Server.HtmlEncode(sdr["cnum"].ToString())+"           ";
            R1.Text = "选课";
            Session["km"] = Server.HtmlEncode(sdr["cname"].ToString());
        }
                
    }    //protected void R1_CheckedChanged(object sender, EventArgs e)
    //{
    //    SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=mytest;Integrated Security=True");
    //    con.Open();
        
        
    //}
    protected void Button1_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=mytest;Integrated Security=True");
        con.Open();
        if (R1.Checked)
        {
            SqlCommand cmd = new SqlCommand("select * from stuxkxx where ID='" + Session["id"].ToString() + "' and cname='" + Session["km"].ToString() + "'", con);            object i = cmd.ExecuteScalar();
            // int i = cmd.ExecuteNonQuery();
            //if (i > 0)
            if (i != null)
            {
                Response.Write("<script>alert('你已经选择了本门课程');</script>");
                // Console.Write("<script>alert('你已经选择了本门课程');</script>");
            }
            else
            {
                //cmd = new SqlCommand("select cname sum(cname) from stuxkxx where cname='"+Session["km"].ToString()+"'", con);
             //从保存学生选课信息表 取出选课人数
                cmd = new SqlCommand("select * from stuxkxx ", con);
                SqlDataReader sdr = cmd.ExecuteReader();
                while (sdr.Read())
                {
                    
                    Session["num"] = sdr["num"].ToString();
                }
                
                string p = Session["num"].ToString();
                sdr.Close();
                //从可参选的课程表 取出课程规定的人数
                cmd = new SqlCommand("select * from xuankebiao where cname='"+Session["km"].ToString()+"'", con);
                sdr = cmd.ExecuteReader();
                while (sdr.Read())
                {                    Session["cnum"] = sdr["cnum"].ToString();
                }
                string q = Session["cnum"].ToString();
                sdr.Close();
                //比较选课人数和课程规定的人数
                if (Session["num"].ToString() == Session["cnum"].ToString())
                {
                    Response.Write("<script>alert('以达到本课程规定的人数 课程不能再选');</script>");
                }
                else
                {                    cmd = new SqlCommand("insert into stuxkxx(ID,Name,cname) values('" + Session["id"].ToString() + "','" + Session["sname"].ToString() + "','" + Session["km"].ToString() + "')", con);
                    //cmd = new SqlCommand("insert into Student(StudentNum,StudentName,StudentPwd,StudentSex) values('" + addstunum.Text.Trim() + "','" + addstuname.Text.Trim() + "','" + 000000 + "','" + addstusex.Text.Trim() + "')", con);
                    int j = cmd.ExecuteNonQuery();
                    if (j > 0)
                    {
                        Response.Write("<script>alert('选课成功');</script>");
                        //Console.Write("<script>alert('选课成功');</script>");
                        Response.Redirect("~/Default2.aspx");                    }
                }
            }
        }
        else
        {
            Response.Write("<script>alert('请点击提交按钮前面的按钮 确认选择');</script>");
        }    }
}
如何从保存学生选课信息表 stuxkxx 中统计出“计算机”这门课的选课人数 并赋值给一个变量 来和可参选的课程表中课程规定的课程人数比较GridViewsql