วอนผู้รู้สอบถามเรื่องการทำ dropdown list กับ jsp ajax ดึงข้อมูลจาก database ที่มีความสัมพันธ์กัน จะต้องทำอย่างไร ค่ะช่วยดูโค้ดให้หน่อยค่ะ pleasessss

<script type="text/javascript">
            var xmlReq ;
function gotoDDL_G () {
                document.frmasset.method = "post";
                document.frmasset.action = "/${strPath}/servlet/SearchServlet?action=DDLG";
                document.frmasset.submit(); 
                
            }
            function selectGroup() {
                var selected_group = document.getElementById("ddlGroup").value; 
                alert(selected_group);
                var complete_url = "/${strPath}/servlet/SelectionServlet?action=TDA&DURART_GROUP_ID="+selected_group;
                if(window.XMLHttpRequest){
                    xmlReq = new XMLHttpRequest();
                }else {
                    xmlReq = new ActiveXObject("Microsoft.XMLHTTP");
                }
                xmlReq.open("GET",complete_url,true);
                xmlReq.send(null);

            }
           function callback(){
                if(xmlReq.readyState === 4){
                    if(xmlReq.status === 200) {
                        var list_type = document.getElementById("ddlType");
                        xmlReq.open("GET","/${strPath}/js/Searching/ddlTypeDurArt.jsp");
                      
                    }
                }
            }

</script>

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package com.rmuti.surin.DurableArticles.servlet;

import com.intcom.utils.StringUtil;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.rmuti.surin.DurableArticles.stringUtil;
import java.util.*;
import com.rmuti.surin.DurableArticles.query.*;
import javax.servlet.RequestDispatcher;
/**
 *
 * @author Asus-A42J
 */
public class SearchServlet extends HttpServlet {

    /**
     * Processes requests for both HTTP
     * <code>GET</code> and
     * <code>POST</code> methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        System.out.println("Run Search Servlet"); 
        String[] values;
        
        //Text Search
        String textSeh = ""; 
        values = request.getParameterValues("textSeh");
        //
        if(values!=null) {
            textSeh = new String(values[0]);
        }
        System.out.println("value textSeh :"+textSeh);
        textSeh = StringUtil.convertToThai(textSeh);
        System.out.println("textSeh Convert :"+textSeh);
        
        //Action 
        String strAction = "";
        values = request.getParameterValues("action"); 
        if(values != null) {
            strAction = new String(values[0]);
        }
        strAction = com.rmuti.surin.DurableArticles.stringUtil.convertToThai(strAction);
        
        //DropDown List GroupID 
        String DURART_GROUP_ID = "";
        values = request.getParameterValues("DURART_GROUP_ID");
        if(values!=null) {
            DURART_GROUP_ID = new String(values[0]);
        }
        System.out.println("value DURART_GROUP_ID  :"+DURART_GROUP_ID);
        textSeh = StringUtil.convertToThai(DURART_GROUP_ID);
        System.out.println("DURART_GROUP_ID Convert :"+DURART_GROUP_ID);
        
        //Action Search 
        if(strAction.equals("SEH")){
        try {
            /* TODO output your page here. You may use following sample code. */
            System.out.println("Sevlet Action SEH"); 
            List lstResult = new ArrayList(); 
            qry_search qrySeh = new qry_search (); 
            lstResult = qrySeh.selectState1(textSeh);
            request.setAttribute("result",lstResult);
            request.setAttribute("lst",Integer.toString(lstResult.size()));
            RequestDispatcher dispatcher = request.getRequestDispatcher("/jsp/Searching/ResultSeh.jsp");
            dispatcher.forward(request, response);
            System.out.println("end Servlet");
             
        } catch (Throwable e) {            
            out.close();
            System.out.println("error  : "+e.toString()); 
            RequestDispatcher dispatcher = request.getRequestDispatcher("/jsp/Searching/search.jsp");
            
        }
        }
        
        //Action Dropdown List Group DurArt
        if(strAction.equals("DDLG")) {
        try {
            System.out.println("Servlet Action DDL_G"); 
            List lstResult = new ArrayList();
            qry_search qrySeh = new qry_search();
            lstResult = qrySeh.selectGroupDA();
            request.setAttribute("result",lstResult);
            request.setAttribute("lst",Integer.toString(lstResult.size()));
            RequestDispatcher dispatcher = request.getRequestDispatcher("/jsp/Searching/search.jsp");
            dispatcher.forward(request, response);
            System.out.println("end Servlet");
        }catch (Throwable e) {
            out.close();
            System.out.println("error : "+e.toString());
            RequestDispatcher dispatcher = request.getRequestDispatcher("/jsp/Searching/search.jsp"); 
        }
        }
        
    }

    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP
     * <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Handles the HTTP
     * <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package com.rmuti.surin.DurableArticles.servlet;

import com.intcom.utils.StringUtil;
import com.rmuti.surin.DurableArticles.query.qry_selections;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.*;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.*;

/**
 *
 * @author Asus-A42J
 */
public class SelectionServlet extends HttpServlet {

    /**
     * Processes requests for both HTTP
     * <code>GET</code> and
     * <code>POST</code> methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        System.out.println("Run Selections Servlet"); 
        String [] values;
        
        //Selected Group 
        //DropDown List GroupID 
        String DURART_GROUP_ID = "";
        values = request.getParameterValues("DURART_GROUP_ID");
        if(values!=null) {
            DURART_GROUP_ID = new String(values[0]);
        }
        System.out.println("value DURART_GROUP_ID  :"+DURART_GROUP_ID);
        DURART_GROUP_ID = StringUtil.convertToThai(DURART_GROUP_ID);
        System.out.println("DURART_GROUP_ID Convert :"+DURART_GROUP_ID);
        
        //Selection Type DurArt
        //Action 
        String strAction = "";
        values = request.getParameterValues("action");
        if(values != null) {
            strAction = new String(values[0]); 
        }
        strAction = com.rmuti.surin.DurableArticles.stringUtil.convertToThai(strAction);
        
        /*
         * DA = Durable Articles
         * T = Type
         * BT = Budget type
         * OF = office 
         * C = campus
         * ORG = organize 
         * sORG = sub oragnize
         * PG = programs 
         * ST = section 
         */
        
        //Action Selection Type Durable Articles
        if (strAction.equals("TDA")){
        try {
            /* TODO output your page here. You may use following sample code. */
            System.out.println("Servlet Action TDA");
            List lstResult = new ArrayList();
            qry_selections ddl_type = new qry_selections();
            lstResult = ddl_type.select_TypeDA(DURART_GROUP_ID);
            request.setAttribute("result",lstResult);
            request.setAttribute("lst",Integer.toString(lstResult.size()));
            RequestDispatcher dispatcher = request.getRequestDispatcher("/jsp/Searching/ddlTypeDurArt.jsp");
            dispatcher.forward(request, response);
            System.out.println("end Servlet");
        } catch (Throwable e) {            
            out.close();
            System.out.println("error : "+e.toString());
            RequestDispatcher dispatcher = request.getRequestDispatcher("/jsp/Searching/Search.jsp");
        }
        }
    }

    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP
     * <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Handles the HTTP
     * <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>
}