> what is the difference between a display only field showing an internal ID 
> number, 
> and a duplicate, display only ID number showing on the entry form?
It really comes down to preference, you can link how you wish. A read only 
numeric ID works in most cases.

The reason I don't use them is that if you expose a read only ID to the 
business, the business owns that ID not you or the application. At that point 
they can request things change around that ID that would/could break your 
linking. Most the time you will be OK, other times it can mess you up or cause 
you to say "No we can't do that" to a client.

As much as I like to think the business is constant and won't change, and that 
unique numbers stay unique, it does happen all of the time, even around read 
only keys. I have been around a long time and have been glad that I use 
internal IDs instead of business exposed keys on multiple occasions.

Using keys that the client never sees, fixes this potential (in a lot of cases 
unlikely) problem. As for me and my databases, I link my data with keys that I 
own and the business can't see.

I also use UUID over longint

 - UUID is faster (do to "random" data in the index)
 - UUID solves problems with distributed systems that sync
 - UUID fixes the home grown sequence problem with transactions
 - UUID is not easily readable by human and keeps me from being tempted to 
expose them :)



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