MySQL: SUM data from two different tables - mysql

Question

I have a db for a menu which tracks clicks on it. The menu has categories and subcategories and I'm trying to get the amount of clicks for each category but in the db, the clicks will register to the subcategory if the item is in one, otherwise the clicks are counted in the category. I have a query that will get clicks for all subcategories (category_type 3) but I need to add them with the clicks from their parent category (category_type 2). There is a table called CategoryHierarchy that maps each category to it's parent category. This is what I have:
SELECT IFNULL(SUM(`MenuEntryAnalytics`.`opened`), 0) AS `clicks`,
`Categories`.`id`,
`Categories`.`name`,
`Categories`.`category_type`,
`CategoryHierarchy`.`parent_id` AS `parent`
FROM `MenuEntryAnalytics`
INNER JOIN `MenuEntries`
ON `MenuEntryAnalytics`.`menu_entry_id` = `MenuEntries`.`id`
LEFT JOIN `MenuEntryToCategory`
ON `MenuEntryAnalytics`.`menu_entry_id` = `MenuEntryToCategory`.`menu_entry_id`
RIGHT JOIN `Categories`
ON `MenuEntryToCategory`.`category_id` = `Categories`.`id`
RIGHT JOIN `CategoryHierarchy`
ON `Categories`.`id` = `CategoryHierarchy`.`category_id`
WHERE `Categories`.`category_type` = 3
GROUP BY `id`;
Results:
clicks id name type parent
=============================================
2032 3 Appetizers 3 2
455 4 Salads 3 2
680 6 Sandwiches 3 5
424 7 Burgers 3 5
584 9 Pizza 3 8
466 10 Kids Menu 3 8
1445 12 Soda 3 11
1089 13 Signature Cocktails 3 11
391 14 Bottled Beer 3 11
167 15 Wine 3 11
0 17 Events 3 16
0 18 Sponsors 3 16
186 19 Dessert 3 11
621 26 Restaurants 3 22
263 27 Bars 3 22
112 28 Services 3 25
254 29 Amenities 3 25
67 30 Exclusive Benefits 3 25
190 31 Area Attractions 3 24
14 32 Entertainment 3 24
2 33 Shopping 3 24
117 34 Transportation & Tours 3 24
471 35 Mixed Drinks 3 11
541 36 Draft Beer 3 11
if I GROUP BY parent then I can get most of what I need (all the clicks from subcategories of each category) but this doesn't get the clicks counted towards categories (as opposed to subcategories, i.e. category_type 2). I'm stuck trying to add that part in, all I can think of is using a subquery but there's no way of identifying which category I'm looking at, thus I get a subquery with multiple rows.
PS I do not have permission to restructure the db.

Answer 1

Since the parent ID is in the same namespace as the ID, you can simply use IFNULL to pick the parent if it exists, or otherwise the ID. And use that as your grouping strategy.
You may also want to select the same data out as an actual column.
GROUP BY
IFNULL(CategoryHierarchy.parent_id, Categories.id, CategoryHierarchy.parent_id)