Its running success but its can be accept the null values, but Im using required filed validation but 1st time its can be accept the null value how to solve the problem

USE [videosample]
GO
/****** Object:  Table [dbo].[Table_1]    Script Date: 05/14/2014 15:53:06 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[Table_1](
	[ID] [int] NULL,
	[VLink] [varchar](max) NULL,
	[UserID] [varchar](50) NULL,
	[LikeCount] [int] NULL,
	[ViewCount] [nchar](10) NULL,
	[UploadDate] [date] NULL,
	[Image] [varchar](max) NULL
) ON [PRIMARY]
GO
SET ANSI_PADDING OFF
GO

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;

public partial class Default2 : System.Web.UI.Page
{
    string s1 = ConfigurationManager.ConnectionStrings["db"].ToString();
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button2_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(s1);
        con.Open();
        SqlCommand cmd = new SqlCommand("insert into Table_1 (userID,VLink,UploadDate,Image)values(@UserID,@VLink,@UploadDate,@Image)", con);
        string s = TextBox2.Text;
        string g = "embed/";
        string o = "watch?v=";
        string m = s.Replace(o, g);
        string[] arrUrl = s.Split('=');
        string h = arrUrl[arrUrl.GetUpperBound(0)];
        string r = "http://img.youtube.com/vi" + h + "/0.jpg";
        cmd.Parameters.AddWithValue("@VLink", m);

        cmd.Parameters.AddWithValue("@UserID", TextBox1.Text);

        cmd.Parameters.AddWithValue("@UploadDate", DateTime.Now);
        cmd.Parameters.AddWithValue("@Image", r);

        cmd.ExecuteNonQuery();
        con.Close();

        Label1.Text = "Upload sucess";
        Label1.ForeColor = System.Drawing.Color.Green;

        TextBox1.Text = "";
        TextBox2.Text = "";




    }
   

   
    protected void DataList1_ItemCommand(object sender, DataListCommandEventArgs e)
    {
        if (Page.IsPostBack)
        {
            if (e.CommandName == "Redirect")
            {

                Session["ID"] = e.CommandArgument;
                Response.Redirect("Default2.aspx");
            }
        }
    }
    protected void DataList1_ItemCommand(object sender, EventArgs e)
    {

    }
}

<asp:Label ID="Label1" runat="server" Text="Enter username"></asp:Label>
        <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
        <br />
        <br />
        <br />
        <asp:Label ID="Label2" runat="server" Text="Enter url"></asp:Label>
        <asp:TextBox ID="TextBox2" runat="server" style="margin-top: 49px"
            Width="357px"></asp:TextBox>
        <asp:Label ID="Label3" runat="server"></asp:Label>
        <br />
        <br />
        <asp:Button ID="Button2" runat="server" onclick="Button2_Click" Text="Button" />
        <br />
        <br />
        <br />
    <asp:DataList id="DataList1" runat ="server" RepeatDirection ="Horizontal"
            RepeatColumns ="3" OnItemCommand ="DataList1_ItemCommand"
            onselectedindexchanged="DataList1_ItemCommand">
    <ItemTemplate>
    <div>
        <table>
                <tr>
                    <td>
                            <asp:LinkButton runat="server" ID="linkbutton1" CommandArgument='<%#Eval("ID") %>' CommandName="Redirect">
                            <asp:Image ID="Image1" ImageUrl='<%#Eval("Image") %>' runat="server" Height="150px" Width="300px" />

                            </asp:LinkButton>

                      </td>
                </tr>
                <tr>
                    <td>
                        posted By:
                        <asp:Label runat="server" ID="sample"><%#Eval("UserID") %></asp:Label>
                        On:
                        <asp:Label runat="server" ID="posteddate"><%#Eval("UploadDate") %></asp:Label>
                    </td>
                </tr>
           </table>
    </div>
    </ItemTemplate>
    </asp:DataList>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;
public partial class Default3 : System.Web.UI.Page
{
    string s1 = ConfigurationManager.ConnectionStrings["db"].ToString();
    protected void Page_Load(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(s1);
        con.Open();
        SqlCommand cmd = new SqlCommand("select VLink from Table_1 where ID='" + Session["ID"].ToString() + "'", con);
        SqlDataReader dr = cmd.ExecuteReader();
        SqlDataAdapter da = new SqlDataAdapter();
        DataSet ds = new DataSet();
        da.SelectCommand = cmd;
        da.Fill(ds);
        con.Close();
        if (ds.Tables[0].Rows.Count > 0)
        {
            DataList1.DataSource = ds;
            DataList1.DataBind();
        }
    }
    protected void DataList1_ItemCommand(object sender, DataListCommandEventArgs e)
    {
        if (Page.IsPostBack)
        {
            if (e.CommandName == "Redirect")
            {

                Session["ID"] = e.CommandArgument;
                Response.Redirect("Default3.aspx");
            }
        }
    }
}

<asp:DataList ID="DataList1" runat="server" RepeatColumns="Horizontal"  OnSelectedIndexChanged="DataList1_ItemCommand">

 <ItemTemplate>
 <iframe runat="server" id="detail" src='<%#Eval("VLink") %>'  height="420px" width="490px"></iframe>
 </ItemTemplate>
 </asp:DataList>