An ID should be system generated and meaningless to the user. It should be used for referential integrity and primary keys. It's generally hidden from the user. If the user needs something special beyond that, add an additional field to support it.

>I use a separate table for the autoinc, because optionally users can use their own numbering.
>Besides, I also have a unique key in the table that is handled differently by the framework, and when using autoinc I was forced to use the numeric key as unique field.
>In the business class it was very easy to use another table to handle the numbering, and gives me more flexibility. When doing straight updates in the SQL database that turns out to be a disadvantage.