急急急请帮助很郁闷
我在前台写了这么个程序
  <form action="LoginServlet" method="post">
  <table border="1" align ="center">
  <tr>
  <td>用户名</td>
  <td> <input type="text" name="username"></td>
  </tr>
  <tr>
  <td>密码</td>
  <td> <input type ="password" name="userpass"></td>
  </tr>
  <tr>
  <td> <input type ="submit" value="登录"> </td>
  <td> <a href="Register.jsp"> 免费注册</a> </td>
  </tr>
  </table>
  </form>

在后台写个这么个类在数据库里明明有三列username,userpass,和sex可为什么总是在结果集里显示只有两列呢请帮忙解答我老郁闷啦
package dljt;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.sql.*;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
 *
 * @author Administrator
 */
@WebServlet(name = "LoginServlet", urlPatterns = {"/LoginServlet"})
public class LoginServlet extends HttpServlet {

  protected void processRequest(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
  request.setCharacterEncoding("UTF-8");
  response.setContentType("text/html;charset=UTF-8");
  PrintWriter out = response.getWriter();
  String username = request.getParameter("username");
  String userpass = request.getParameter("userpass");
  Connection conn = null;
  Statement st = null; 
  ResultSet rs = null;
  String Driver = "org.apache.derby.jdbc.ClientDriver";
  String url = "jdbc:derby://localhost:1527/sample";
  String user = "app";
  String pass = "app";
  String sql = "select username,userpass from QQ where username = '"+username+"'and userpass ='"+userpass+"'";
   
  try {
  Class.forName(Driver);
  conn = DriverManager.getConnection(url, user, pass);
  st = conn.createStatement();
  rs = st.executeQuery(sql);
  if(rs.next()) {
  System.out.println(rs.getString(3))在这出的问题
  out.println(rs.getString("USERNAME")+" "+rs.getString("USERPASS")+" ");
  }else {
  out.println("<script language = Javascript>");
  out.println("window.alert(\"用户不存在请注册\")");