c# - 如何从C#中的查询字符串中获取列值

Question

我想知道表中的数量列是否有 NULL 值，如果它有 NULL 值，那么我想做一个操作，如果没有，那么我想做一些其他操作

有人可以帮我解决这个问题吗？

我试过这个代码：

protected void Page_Load(object sender, EventArgs e)
 {
        if (!this.IsPostBack)
        {

            string select = "select * from business where category_id=23 ";
            scon = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=D:\bookdb.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True");
            scon.Open();
            DataTable table = new DataTable();
            cmd = new SqlCommand(select, scon);
            dr = cmd.ExecuteReader();
            int count = 1;

            while (dr.Read())
            {
                TableRow rnew = new TableRow();
                Table1.Rows.Add(rnew);

                TableCell cnew = new TableCell();
                TableCell cnew1 = new TableCell();
                Label lb1 = new Label();
                lb1.Text = "(" + count.ToString() + ")<br/>";

                Image img = new Image();
                img.ImageUrl = dr["image_url"].ToString();
                Label lb = new Label();
                lb.Text = "Name:-" + dr["book_name"].ToString() + "<br/>";
                lb.Text += "Author:-" + dr["author"].ToString() + "<br/>";
                lb.Text += "price:-" + dr["price"].ToString() + "Rs.<br/><br/>";
                lb.Text += dr["notes"].ToString() + "<br/><br/><br/>";

                foreach(DataRow row in table.Rows)
                {
                object value = row["quantity"];
                if (value == DBNull.Value)
                LinkButton lbt = new LinkButton();
                lbt.Text = "Order";
                lbt.PostBackUrl = "~/orderlogin.aspx?itemid=" + dr["item_id"].ToString() + "&" + "catid=" + dr["category_id"].ToString();
                lbt.visible=true;
                else
                lbt.visible=false;
                }
                cnew.Controls.Add(lb1);
                cnew.Controls.Add(img);
                cnew.Controls.Add(lbt);

                cnew1.Controls.Add(lb);

                rnew.Cells.Add(cnew);
                rnew.Cells.Add(cnew1);
                count++;
            }
            dr.Close();
            scon.Close();

        }
    }
}