ขอสอบถามเรื่องของการ Count Product แยกตาม Category หน่อยครับ

SELECT a.category_name , COUNT(b.id) AS Num FROM Category a
LEFT JOIN Product b ON a.id  = b.category_id
GROUP BY a.category_name

function get_cate($parent, $level) {
	$result = mysql_query("SELECT c.category_id, c.category_name, d.Count FROM category c  LEFT OUTER JOIN (SELECT category_parent, COUNT(*) AS Count FROM category GROUP BY category_parent) d ON c.category_id = d.category_parent WHERE c.category_parent=" . $parent);

	while ($row = mysql_fetch_assoc($result)) {
		$arr = mysql_fetch_assoc(mysql_query("select count(p.category_id) as num_product from product p where p.category_id='$row[category_id]'"));
		
		if ($row['Count'] > 0) {
			echo "<tr><td><a href='" . $row['category_name'] . "'>" . $row['category_name'] . "</a>";
			get_cate($row['category_id'], $level + 1);
			echo "</td><td>".$arr['num_product']." ชิ้น</td></tr>";
		} elseif ($row['Count']==0) {
			echo "<li><a href='" . $row['category_name'] . "'>" . $row['category_name'] . "</a> ".$arr['num_product']." ชิ้น</li>";
		} else;
	}

}
echo get_cate(0, 1);	

function retrieveSubTree($parent, $myarray) {
    $tempArray = $myarray;
    $array = array();           
    //now we have our top level parent, lets put its children into an array, yea!
    while ($child = array_search($parent, $tempArray)) {
        unset($tempArray[$child]);
        //now lets get all this guys children
        if (in_array($child, $tempArray)) {
            $array[$child] = retrieveSubTree($child, $tempArray);
        } else {
            $array[$child] = true;
        }
    }//end while
    return (!empty($array)) ? $array : false;
}

function retrieveTree($myarray) {
    $array = array();
    $counter = 0;
    foreach ($myarray as $key => $value) {
        $child = $key;
        $parent = $value;
        //if this child is a parent of somebody else
        if (in_array($child, $myarray) && $parent != '0') {
            while ($myarray[$parent] != '' && $myarray[$parent] != '0') {
                $newparent = $myarray[$parent];
                $parent = $newparent;
            }
            if (!array_key_exists($parent, $array)) {
                $array[$parent] = retrieveSubTree($parent, $myarray);
            }
        } else {
            //now make sure they don't appear as some child
            if (!array_key_exists($parent, $myarray)) {
                //see if it is a parent of anybody
                if (in_array($child, $myarray)) {
                    $array[$child] = retrieveSubTree($child, $myarray);
                } else {
                    $array[$child] = true;
                }
            }//end if array key
        }//end initial in array
    }//end foreach
    return (!empty($array) ? $array : false);
} 

$test = array(
    '1'=>'15',
    '2'=>'1',
    '3'=>'1',
    '4'=>'0',
    '5'=>'0',
    '6'=>'4',
    '7'=>'6',
    '8'=>'7',
    '9'=>'2',
    '10'=>'9'
);