试图获取父母ID的父母ID

Question

I am working on an e-commerce site from scratch using PHP and MYSQL . I have one table for categories, with a column for ID and a column for Parent_ID .

When displaying the products on categories.php , I have it set to display products where the Parent_ID OR the ID equals the $_GET['id'] field. But now I've run into a problem.

In my "Groceries" category, I have the "Cleaning & Home" subcategory, and under "Cleaning & Home" I have several categories like "Laundry", "Bathroom Supplies", etc.

My problem is that products in the third level don't display in the "Groceries" category, because technically the parent ID of "Laundry" is "Cleaning & Home". There will never be more than three levels (Parent, child, grandchild), but I would like categories in the grandchild level to also display in the parent level.

I've tried looking through MYSQL documentation and on other forums but so far no luck.

Answer 1

This requires a couple of joins to get to the top parent:

select c.*,
       coalesce(cp2.id, cp.id, p.id) as MostParentId
from categories c left outer join
     categories cp
     on c.parent_Id = cp.id left outer join
     categories cp2
     on cp.parent_id = cp2.parent_id
where c.id = $_GET['id'] or cp.id = $_GET['id'] or cp2.id = $_GET['id'];

You can then compare the id s using or for a match to the parent category, subcategory, or whatever.