添加用户时 在后台判断此用户是否已经存在

//添加用户
 private void addUser(HttpServletRequest req,HttpServletResponse res) throws ServletException,IOException, SQLException{
  db d=new db();
  Connection conn=d.getConn();
  String userid=PKGenerator.ReceiveGUID();
  String userName=req.getParameter("dlname");
  String sql2="select count(*) num from users where userName='"+userName+"'";
  ResultSet rs=d.executeQuery(sql2);
  int num=0;
  if(rs!=null&&rs.next()){
   num=rs.getInt("num");
  }
  if(num>0){
   res.setContentType("text/html;charset=utf-8");
   res.setHeader("Cache-Control", "no-cache");
   res.getWriter().write("<script  type='text/javascript'>alert('此用户已存在');history.go()</script>");
  }else{
  String password=req.getParameter("password");
  String name=req.getParameter("name");
  String email=req.getParameter("email");
  String phone=req.getParameter("phone");
  String gpid=req.getParameter("gpid");
  int type=new tongyong().isNotNull(req.getParameter("type"))? Integer.parseInt(req.getParameter("type")):0;
  
  String sql="insert into users (userid,userName,password,name,email,phone,type) values (?,?,?,?,?,?,?)";
  PreparedStatement ps=conn.prepareStatement(sql);
  int i=1;
  ps.setString(i++,userid);
  ps.setString(i++, userName);
  ps.setString(i++, password);
  ps.setString(i++, name);    
  ps.setString(i++, email);
  ps.setString(i++, phone);
  ps.setInt(i++, type);
  
  ps.executeUpdate();
  ps.close();
  conn.commit();
  conn.close(); 
  res.sendRedirect("../main/yonghulist.jsp");
  }
 }