How to update MySQL database with Ajax + PHP

I need to do an update to my MySQL database asynchronously.

I have a table with bank results.

<?php 
        $sqlCategoria = "SELECT * FROM categorias";
        $resultCategoria = mysql_query($sqlCategoria);
        while($categorias = mysql_fetch_array($resultCategoria)){ 
?>

No while, I put this script to update.

But it only works the first time.

<script type="text/javascript">

function alterar_div() {
  $.ajax({
    type: "POST",
    url: "processo.php",
    data: {
        id_item: $('#id_item').val(),
        valor: $('#valor').val()
    },
    success: function(data) {
      $('#conteudo').html(data);
    }
  });
}

</script>

Why is this happening?

The Code as I Did!

<script type="text/javascript"> 
function alterar_div() {
  $.ajax({
    type: "POST",
    url: "processo.php",
    data: {
        id_item: $('#id_item').val(),
        valor: $('#valor').val()
    },
    success: function(data) {
      $('#conteudo').html(data);
    }
  });
}
</script>

<?php 
    $sqlCategoria = "SELECT * FROM categorias";
    $resultCategoria = mysql_query($sqlCategoria);
    while($categorias = mysql_fetch_array($resultCategoria)){ 

        $sqlItens ="SELECT * FROM itens WHERE it_ctid = '".$categorias['ct_id']."'";
        $resultItens = mysql_query($sqlItens);
        while($itens = mysql_fetch_array($resultItens)){

            $sqlNao = "select rp_valor, rp_id from respostas
                            left join check_list on (ch_id = rp_chid)
                            left join itens on (it_id = rp_itid)
                            where ch_token = '".$token."' and it_id = '".$itens['it_id']."'";

            $resultNao = mysql_query($sqlNao);
            $valor = mysql_fetch_array($resultNao);

             }?>

    <input id="id_item" type="hidden" value="<? echo $valor['rp_id']; ?>" />
    <input id="valor" type="hidden" value="0" />
    <button type="button"  onclick="alterar_div()"> <img src="assets/imagens/bt_nao2.png" width="30" height="30" alt=""/></button>

<?php }?>

Process.pho file

<?php

include('includes/mysql.php');

$id = $_POST['id_item'];
$valor = $_POST['valor'];

$sql = mysql_query("UPDATE respostas SET rp_valor = '".$valor."' WHERE rp_id = '".$id."'");

?>