insert data from one Table into other Table if data does not exists in SQL Server

HI

I have 2 table in database

Estate_p Table

and Advertisment Tabel

and I have 2 textbox and one imagebutton  and one label (that show error message)  and one gridview in my page

in txtcode users enter code

and in txtad enter number

I want when users click on Imagebutton if there exist  code( In  Estate_p code column)  (that users enter code in txtcode) when they click on button it insert data from estate_p into Advertisment tabel and if there wasn't code in estate_p tabel it show error that i define in lable:

I wrote below code

protected void  ImageButton1_Click(object sender, ImageClickEventArgs e)
{


    string data = Server.UrlDecode(Request.QueryString["BehCode"]);

    SqlCommand _cmd = new SqlCommand("insertAD", _cn);
    _cmd.CommandType = CommandType.StoredProcedure;
    _cn.Open();
    _cmd.Parameters.AddWithValue("@behcode", data);
    _cmd.Parameters.AddWithValue("@code", txtcode.Text);
    _cmd.Parameters.AddWithValue("@day", txtad.Text);
    int Result = _cmd.ExecuteNonQuery();
    if (Result > 0)
    {

       
        lblerrorV.Visible = false;

    }
    else
    {
        lblerrorV.Visible = true;
    
    }

  

    _cn.Close();


    BindData();
    txtcode.Text = " ";
    txtad.Text = " ";

and SP

USE [behtop]
GO
/****** Object:  StoredProcedure [dbo].[insertAD]    Script Date: 11/10/2012 19:29:16 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER procedure [dbo].[insertAD]

@day int
,@behcode nvarchar(10)
,@code varchar(20)
as
begin
set NOCOUNT ON;
 insert into advertisment(behcode,date
Name,days)




select behcode,date,name ,@day

from Estate_p 
 where behcode=@behcode and Code=@code
 end

when I enter wrong code in txtcode it didn't insert any thing in tabel and show error label but after that when I enter right code it again didn't insert any thing in tabel and show error label too

I want when users enter right code it insert data from estate_p tabel into Advertisment tabel and if usersenter wrong code it didn't insert data and show laberror

Best regards