Flatten Adjacency List Hierarchy To A List Of All Paths

Question

I have a Table that stores Hierarchical information using the Adjacency List model. (uses a self referential key - example below. This Table may look familiar):

<         


        

Answer 1

To do multi-level queries across a simple adjacency-list invariably involves self-left-joins. It's easy to make a right-aligned table:

SELECT category.category_id,
    ancestor4.category_id AS lvl4,
    ancestor3.category_id AS lvl3,
    ancestor2.category_id AS lvl2,
    ancestor1.category_id AS lvl1
FROM categories AS category
    LEFT JOIN categories AS ancestor1 ON ancestor1.category_id=category.category_id
    LEFT JOIN categories AS ancestor2 ON ancestor2.category_id=ancestor1.parent
    LEFT JOIN categories AS ancestor3 ON ancestor3.category_id=ancestor2.parent
    LEFT JOIN categories AS ancestor4 ON ancestor4.category_id=ancestor3.parent;

To left-align it like your example is a bit more tricky. This comes to mind:

SELECT category.category_id,
    ancestor1.category_id AS lvl1,
    ancestor2.category_id AS lvl2,
    ancestor3.category_id AS lvl3,
    ancestor4.category_id AS lvl4
FROM categories AS category
    LEFT JOIN categories AS ancestor1 ON ancestor1.parent IS NULL
    LEFT JOIN categories AS ancestor2 ON ancestor1.category_id<>category.category_id AND ancestor2.parent=ancestor1.category_id
    LEFT JOIN categories AS ancestor3 ON ancestor2.category_id<>category.category_id AND ancestor3.parent=ancestor2.category_id
    LEFT JOIN categories AS ancestor4 ON ancestor3.category_id<>category.category_id AND ancestor4.parent=ancestor3.category_id
WHERE
    ancestor1.category_id=category.category_id OR
    ancestor2.category_id=category.category_id OR
    ancestor3.category_id=category.category_id OR
    ancestor4.category_id=category.category_id;

would work for n-tier hierarchies.

Sorry, arbitrary-depth queries are not possible in the adjacency-list model. If you are doing this kind of query a lot, you should change your schema to one of the other models of storing hierarchical information: full adjacency relation (storing all ancestor-descendent relationships), materialised path, or nested sets.

If the categories don't move around a lot (which is usually the case for a store like your example), I would tend towards nested sets.