Edit dynamic table row - php

Question

I'm trying to edit the form that created dynamically. but when I checked the checkbox(s), fill the form and click Edit button, only edited contents show up. The rests are all gone.
The table:
<table class="table" id="list">
<thead>
<tr>
<th>Product</th>
<th>Qty</th>
<th></th>
</tr>
</thead>
<tbody>
</tbody>
</table>
Inputs, Add and Edit buttons:
<input type="text" name="product" id="p" />
<input type="text" name="qty" id="q" />
<button type="button" id="add">Add</button>
<button type="button" id="edit">edit</button>
Script:
<script>
$(document).ready(function(){
$('#add').click(function(){
var product = $('#p').val();
var qty = $('#q').val();
data = 'product='+product+'&qty='+qty+'&action=add';
$.ajax({
url:'add.php',
type:'POST',
data:data,
success:function(data){
$('#list tbody').prepend(data);
}
});
});
$('#edit').click(function(){
var product = $('#p').val();
var qty = $('#q').val();
data = 'product='+product+'&qty='+qty+'&action=edit';
$.ajax({
url:'edit.php',
type:'POST',
data:data,
success:function(data){
$('#list tbody').html(data);
}
});
});
});
</script>
add.php
if($_POST['action']=='add'){
$p=$_POST["product"];
$q=$_POST["qty"];
echo '<tr>';
echo '<td>'.$p.'</td>';
echo '<td>'.$q.'</td>';
echo '<td><input type="checkbox" name="" value="" /></td>';
echo '</tr>';
}
edit.php
if($_POST['action']=='edit'){
$p=$_POST["product"];
$q=$_POST["qty"];
echo '<tr>';
echo '<td>'.$p.'</td>';
echo '<td>'.$q.'</td>';
echo '<td><input type="checkbox" name="" value="" /></td>';
echo '</tr>';
}
I think I should assign an id to echo '<tr id="x">' but I don't know how.
Any advice would be greatly appreciated.
John