public function createCategory($parent_id, $category_name) {
        $sql = "
            LOCK TABLE categories WRITE;
            SELECT @parentDepth :=category_depth FROM categories WHERE category_id = :parent;
            SELECT @myLeft := category_left FROM categories WHERE category_id = :parent;
            UPDATE categories SET category_right = category_right + 2 WHERE category_right > @myLeft;
            UPDATE categories SET category_left = category_left + 2 WHERE category_left > @myLeft;
            INSERT INTO categories(category_name, category_left, category_right, category_parent,category_depth)
            VALUES(:name, @myLeft + 1, @myLeft + 2, :parent,@parentDepth+1);
            UNLOCK TABLES;";
        $params = array(
            ':parent' => array(
                'param' => $parent_id,
                'type' => 1
            ),
            ':name' => array(
                'param' => $category_name,
                'type' => 2
            )
        );
        return $this->_query->query($sql, $params);
    }