如何对去重后的MySQL分类数据按多字段排序并忽略NULL值？

需要获取按siteId去重后的分类列表，并按指定顺序排列。

数据表结构 #

categories
=============
categoryId,
siteId,
parentId,
title,
active

当前使用的SQL语句 #

SELECT
parentTitle,
childTitle,
subChildTitle
from (
    select      subChild.categoryId as parentCategoryId,
                child.categoryId as childCategoryId,
                parent.categoryId as subChildCategoryId,
                subChild.title as subChildTitle,
                child.title as childTitle,
                parent.title as parentTitle
    from        categories subChild
    left join   categories child on child.categoryId = subChild.parentId 
    left join   categories parent on parent.categoryId = child.parentId 
    where subChild.siteId in (1,2,3)
    and subChild.active = 'Y'
    ) as categories 
group by parentTitle, childTitle, subChildTitle
order by COALESCE(parentTitle, childTitle, subChildTitle);

当前查询结果 #

NULL       NULL         Appliance   
NULL       Appliance    Dishwasher  
NULL       Appliance    Dryer
Appliance  Dishwasher   Not Cleaning Correctly
Appliance  Dryer        Not Cleaning

目标排序结果（优先实现） #

NULL       NULL         Appliance   
NULL       Appliance    Dishwasher  
Appliance  Dishwasher   Not Cleaning Correctly
NULL       Appliance    Dryer
Appliance  Dryer        Not Cleaning

更优目标结果（可选） #

Appliance     NULL        NULL
Appliance     Dishwasher  NULL
Appliance     Dishwasher  Not Cleaning Correctly
Appliance     Dryer       NULL
Appliance     Dryer       Not Cleaning

内容的提问来源于stack exchange，提问作者danjfoley