<a href="/RegisterEvent?id=<%=Integer.parseInt(resultSet.getString("event_id")) %>">Register</a>

package com.event;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.*;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.sql.*;
import java.io.*;
import java.util.*;
import javax.servlet.annotation.MultipartConfig;
import javax.servlet.http.HttpSession;
import javax.servlet.http.Part;

public class RegisterEvent extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        int i1;

        HttpSession session = request.getSession();
        String username = (String) session.getAttribute("uname");

        String event_id = request.getParameter("id");

        String fname, lname, email, colgname, event_name;
        int contact;

        try {

            //loading drivers for mysql
            Class.forName("com.mysql.jdbc.Driver");

            //creating connection with the database
            Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/technovision", "root", "root");

            String sql1 = "SELECT * FROM `users` WHERE `username`=?";

            PreparedStatement ps = con.prepareStatement(sql1);

            ps.setString(1, username);

            ResultSet rs = ps.executeQuery();

            while (rs.next()) {
                fname = rs.getString("fname");
                lname = rs.getString("lname");
                email = rs.getString("emailid");
                contact = rs.getInt("mobile");
                colgname = rs.getString("colgname");
            }

            String sql2 = "SELECT event_name FROM events WHERE event_id=?";

            PreparedStatement ps1 = con.prepareStatement(sql2);

            ps1.setInt(1, Integer.parseInt(event_id));

            ResultSet rs1 = ps1.executeQuery();

            while (rs1.next()) {
                event_name = rs1.getString("event_name");
            }

            String sql3 = "INSERT INTO registered_users(`fname`,`lname`,`email`,`contact`,`college_name`,`event_name`) values (?,?,?,?,?,?)";

            PreparedStatement ps3 = con.prepareStatement(sql3);

            ps3.setString(1, fname);
            ps3.setString(2, lname);
            ps3.setString(3, email);
            ps3.setInt(4, contact);
            ps3.setString(5, colgname);
            ps3.setString(6, event_name);

            i1 = ps3.executeUpdate();

            if (i1 > 0) {
                response.sendRedirect("register-success.jsp");
            }

        } catch (Exception se) {
            System.out.println(se);
        }

    }
}

String fname = "", lname = "", email = "", colgname = "", event_name = "";

RegID EventId userId
1     1       X