Storing route in DB

Hi, I am not sure if my issue can be partially solved by a good database design or that all stands or falls with the application. If I have a bunch of locations in a table and I want to describe a route between a number of these locations, what's the best way to do it, considering that it must be possible to change the route afterwards. For instance (A-B-C-A, becomes A-C-B-A). Ofcourse it's not a really big problem to solve in PHP. However I believe it would be best if the database enforced some rules about how a route may look. For instance no matter what any user or application does, it should not be possible to store a route that looks like A-B-B-A or like A-B- -A or lose all route information when there are still bookings. Logically the problem is not too difficult, however I am looking for a way to implement this in MySQL/PHP. Any pointers to help me in my analysis would be much appreciated. Bob