public partial class Register : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        string username = txtUsername.Text;
        string FirstName = txtFirstName.Text;
        string LastName = txtLastName.Text;
        string Password = txtPassword.Text;
        string ConfirmPassword = txtPassword1.Text;
        int Date = int.Parse(txtDate.Text);
        string Countries = txtCountries.Text;

        string sql = string.Format("SELECT * FROM Register WHERE Username='{0}'", username);
        string con = "Data Source=(LocalDB)\\MSSQLLocalDB;";
        con += "AttachDbFilename =|DataDirectory|\\Database.mdf;";
        con += "Integrated Security=True;";



        SqlConnection connection = new SqlConnection(con);
        connection.Open();
        SqlCommand command = new SqlCommand(sql, connection);
        SqlDataReader reader = command.ExecuteReader();
        if (!reader.HasRows)
        {
            reader.Close();
            sql = "INSERT INTO Register(Username, FirstName, LastName, Password, ConfirmPassword, Date, Countries)";
            sql += string.Format("VALUES('{0}', '{1}', '{2}', '{3}', '{4}', {5}, '{6}')", username, FirstName, LastName, Password, ConfirmPassword, Date, Countries);
            command = new SqlCommand(sql, connection);
            command.ExecuteNonQuery();
            Response.Redirect("Planes.html");
        }
        else
        {
            lblError.Text= "User already exists";
        }
    }
}