A good algorithm for generating an order number

Question

As much as I like using GUIDs as the unique identifiers in my system, it is not very user-friendly for fields like an order number where a customer may have to repeat that to a

Answer 1

The straightforward answer to most of your bullet points:

Make the first six digits a sequentially-increasing field, and append three digits of hash to the end. Or seven and two, or eight and one, depending on how many orders you envision having to support.

However, you'll still have to call a function on the back-end to reserve a new order number; otherwise, it's impossible to guarantee a non-collision, since there are so few digits.