<label>大类：</label> 
<select name="bigname" id="bigname"> 
  <option value="1">前端技术</option> 
  <option value="2">程序开发</option> 
  <option value="3">数据库</option> 
</select> 
<label>小类：</label> 
<select name="smallname" id="smallname"> 
<option value="1">flash</option> 
<option value="2">ps</option> 
</select> 

function getSelectVal(){ 
  $.getJSON("server.php",{bigname:$("#bigname").val()},function(json){ 
    var smallname = $("#smallname"); 
    $("option",smallname).remove(); //清空原有的选项 
    $.each(json,function(index,array){ 
      var option = "<option value='"+array['id']+"'>"+array['title']+"</option>"; 
      smallname.append(option); 
    }); 
  }); 
} 

smallname.(); 

$(function(){ 
  getSelectVal(); 
  $("#bigname").change(function(){ 
    getSelectVal(); 
  }); 
}); 

include_once("connect.php"); //链接数据库 
 
$bigid = $_GET["bigname"]; 
if(isset($bigid)){ 
  $q=mysql_query("select * from catalog where cid = $bigid"); 
  while($row=mysql_fetch_array($q)){ 
    $select[] = array("id"=>$row[id],"title"=>$row[title]); 
  } 
  echo json_encode($select); 
} 

CREATE TABLE `catalog` ( 
 `id` mediumint(6) NOT NULL auto_increment, 
 `cid` mediumint(6) NOT NULL default '0', 
 `title` varchar(50) NOT NULL, 
 PRIMARY KEY (`id`) 
) ENGINE=MyISAM DEFAULT CHARSET=utf8;