Post value using ajax to function autocomplete - php

Question

I have here a dropdown select and autocomplete function. What I need to do is to pass the selected value of dropdown to autocomplete.php to use in my query. Textbox value should depending on value from dropdown. If selected value is supplies, all supplies only value in textbox (like pencil or ballpen).
I used this Ajax in Dynamic Dropdown. How I can use this to pass the value in autocomplete.php?
Note: this Ajax was not connected in my autocomplete function. How I can use this ajax to pass the value to my autocomplete.php query.
<script type="text/javascript">
$('#main').change(function(){
$.ajax({
url : 'getajax.php',
data :{mainlist_id : $(this).val()},
dataType:'html',
type:'POST',
success:function(data){
$('#sub').html(data);
}
});
});
</script>
Ajax.php
<script type="text/javascript" src="jquery.autocomplete.js"></script>
<script>
$(document).ready(function(){
$("#tag").autocomplete("autocomplete.php", {
selectFirst: true
});
});
</script>
Drop1
<?php
$combo = $mysqli->query("SELECT * FROM category GROUP BY cat_code ORDER BY id");
$option = '';
while($row = $combo->fetch_assoc())
{
$option .= '<option value = "'.$row['cat_code'].'">'.$row['category'].'</option>';
}
?>
<select id="main" name="main">
<option value="" disabled="disabled" selected="selected">Choose</option>
<?php echo $option; ?>
</select>
Auto Complete <input id="tag" type="text">
Autocomplete.php
<?php
$mysqli = new mysqli("localhost", "root", "", "2015") or die("Database Error");
$auto = $mysqli->real_escape_string($_GET["q"]);
$sql = $mysqli->query("SELECT * FROM code WHERE item LIKE '%$auto%' GROUP BY id ORDER BY item" );
if($sql)
{
while($row=mysqli_fetch_array($sql))
{
echo $row['item']."\n";
}
}
?>

Answer 1

$('#sub').html(data); should be $('#tag').html(data);

Answer 2

You have not defined #sub element in your html code, when placing this
$('#sub').html(data);
After that your name of the page must be same, while your using getajax.php, and its not you have defined us