获取php页面中的参数，该页面由ajax函数调用

我正在使用jquery和ajax调用一个php页面res.php。代码是：-

$('#submit_button').click(function () {        
    buildingVal = $("#building");
    levelVal = $("#level");
    data = 'building=' + buildingVal.val() + 'level=' + levelVal.val();
    $.ajax( {
        url: "res.php", 
        type: "POST",
        data: data,     
        success: function (data) {
            $('#npc').html(data);
        }
    });​
});

res.php页面代码为：-

<?php
//connect to the database
$con = mysql_connect("localhost","root","12345") or die("error ".mysql_error());
//connect to the travian table
mysql_select_db("trav",$con) or die("error ".mysql_error());
$building = mysql_real_escape_string($_GET['building']);
$level = mysql_real_escape_string($_GET['level']);

$query = "select * from ";
$query = $query . $building;
$query = $query . "where lvl=" . $level;
$query = $query . ";";
$result = mysql_query($query) or die('Error in Child Table!');
$data = mysql_fetch_assoc($result);
echo '<table><tr><td>Lumber=$data["lumber"]</td><td>Clay=$data["clay"]</td><td>Iron=$data["iron"]</td><td>Crop=$data["crop"]</td>';
?>

我收到错误

Notice: Undefined index: building in C:'xampp'htdocs'debal'res.php on line 8
Notice: Undefined index: level in C:'xampp'htdocs'debal'res.php on line 9
Error in Child Table!

我该如何提取发送到页面的两个参数，并在sql查询中使用它们从数据库中的表中检索数据。你能帮我吗？