asp.net gridview 删除CheckBox选中记录无反应（页面只是刷新一下，没有删除）

删除按钮代码
 string sql = "";
        string connString = ConfigurationManager.ConnectionStrings["sjlj"].ToString();
        SqlConnection conn = new SqlConnection(connString);
        for (int i = 0; i <= GridView1.Rows.Count - 1; i++)
        {
            CheckBox cbox = (CheckBox)GridView1.Rows[i].FindControl("CheckBox1");
            if (cbox.Checked == true)
            {
                sql = "delete from rkb where id='" + GridView1.DataKeys[i].Value + "'";
                conn.Open();
                SqlCommand com = new SqlCommand(sql, conn);
                com.ExecuteNonQuery();
                conn.close();

            }
        }
        sjdq();
数据库读取代码
public void sjdq()
    {
        string connString = ConfigurationManager.ConnectionStrings["sjlj"].ToString();
        SqlConnection con = new SqlConnection(connString);
        con.Open();
        string sql = "select * from rkb order by 日期";
        SqlDataAdapter da = new SqlDataAdapter();
        da.SelectCommand = new SqlCommand(sql, con);
        DataSet ds = new DataSet();
        da.Fill(ds);
        GridView1.DataSource = ds;
        GridView1.AllowPaging = true;
        GridView1.PageSize = 5;
        GridView1.DataKeyNames = new string[] { "id" };
        GridView1.DataBind();
        con.Close();
    }