Using null to mean all or any

[Jim Haberlin]

I am new to MySQL and I am trying to determine how to handle a situation where I have a foreign key but I want to have a hierarchy and have a meaning of ALL or ANY.

These easiest way to describe it is with an example.

I have a customer table with many customers.
I have a rate table with many rates.
I want to set up a rate of $100.00 that applies to all of my customers.
I then need a rate of $90.00 that applies to only one customer, Customer A.
If I am searching for the correct rate for Customer A I should find the row with the $90.00 rate.
If I search for a rate for any other customer I should find the rate of $100.00.

My instinct is to have a customer foreign key on the rate table that would have Customer A's key on the $90.00 rate and a null value on the $100.00 rate.
The query would be: Select amount from Rate where rate.customer is "Customer A" or rate.customer is null.

This works but I am concerned with efficiency and with having correct database design. I am not arguing in favor of this solution I am merely trying to find out if it is a valid design or if there are other solutions for this scenario.

I would appreciate any suggestions.