SQL Table Design Opinion

I have a table that is responsible for holding data relating to categories and subcategories -- let's say, as an example, songs by band based by genre. I'm using a multi-key table to hold that information:


songs
+-------------+----------------+------------------------+
| category_id | subcategory_id | song                   |
+-------------+----------------+------------------------+
| 1           | 1              | All My Lovin'          |
| 1           | 2              | Sympathy for the Devil |
+-------------+----------------+------------------------+

...where category_id = 1 might be 60's music, and subcategory = 1 is The Beatles.

So, let's say I have base prices that I want to attach to these categories -- a standard base price that applies to every song in a given category. I would have a table that relates to categories like so:


category_prices
+-------------+-----------+
| category_id | baseprice |
+-------------+-----------+
| 1           | 0.99      |
+-------------+-----------+

But let's say I want to have base prices on the subcategory. Let's say I want those base prices to override base prices set by category. How would you design this table?

Right now I appear to have two options.

The first (and most obvious) is to create two tables. The second table would have two keys: category_id and subcategory_id. Overriding would happen when you set up the JOIN.

The second is to put it all in the same table. For base prices that are only applying to category, I would make the subcategory_id = 0. Overriding would happen with a precise SELECT.

Either way works fine, so at this point I'm simply curious. What would you do?

>>3
That's what I thought, too. I really would have preferred to use null, but I had problems using null as a key, I guess because null is not UNIQUE by definition. But it's possible I'm just setting up the table wrong. I feel really uncomfortable using 0, it seems to me like a problem just waiting to happen.

(What I mean: category_id and subcategory_id are both keys. That way you can get things like


categories
+-------------+--------------+
| category_id | name         |
+-------------+--------------+
| 1           | 60's rock    |
| 2           | 80's hair    |
+-------------+--------------+
subcategories
+-------------+----------------+---------------+
| category_id | subcategory_id | name          |
+-------------+----------------+---------------+
| 1           | 1              | The Beatles   |
| 1           | 2              | The Stones    |
| 1           | 3              | Cream         |
| 2           | 1              | Poison        |
| 2           | 2              | Whitesnake    |
+-------------+----------------+---------------+

...which seemed conceptually accurate to my needs.)