My website currently organizes Articles by "Section", and to further divide things, I want to introduce the concept of a "Sub-Section".

However, I want to ensure that any given "Sub-Section" can only ever be associated with one parent "Section".

For example, this is okay...
Code:
Section		Sub-Section
--------	------------
Finance		Economy
Finance		Markets
Finance		Investing

Legal		Business Structure
Legal		Intellectual Property

But this is NOT okay...
Code:
Section		Sub-Section
--------	------------
Finance		Economy

Legal		Economy

As I see it, there is a simple "one-to-many" relations between my "Section" and "Sub-Section" tables.

Code:
Section -||------|<- SubSection

How do I enforce this Business Constraint (in MySQL) mentioned above?

Sincerely,


Debbie