Say I have a table with three columns - description, parent_id, and
id. Each parent_id can, itself, point to another id with a
parent_id. It doesn't have to, but it can. If it doesn't, parent_id
will be null.

As such, if you get one id, you can sorta build a list, by looking at
each parent_id's parent_id until parent_id is null.

My question is... is there a way you can get a list of all the
description's with just a single SQL query?

You can get one description by doing this:

SELECT description FROM table WHERE id = $id

You can get two by doing this:

SELECT a.description, b.description FROM table a, table b WHERE a.id =
b.parent_id AND b.id = $id

....and so on and so forth.

The problem with this is that each "depth" has it's own query and
requires you know the "depth" in advance. Also, I don't imagine doing
dozens of implicit joins, like that, is all that efficient.

Is there a way to do this using a single query that doesn't require
you know the "depth" in advance?

1) please learn to post DDL in newsgroup and not your personal
narratives
2) Google up "Nested Sets Model" and use it instead of this adjacency
list model
3) Buy a copy of TREES & HIERARCHIES IN SQL