>I have made auto-incrementing keyfields before, using the system in TASTRADE. What would be a good system for auto-incrementing while having each "digit" cycle through a-z, or maybe 0-9 and a-z, or maybe A-Z and then a-z? Doing this would increase the number of available values. It would be useful to me because I have a legacy app, written using data from other people's legacy apps, which has a meaningful keyfield which is only five characters long, and always starts with a digit. Now they want to allow blank values in this field, and we don't feel like rewriting the whole thing. I propose to generate values always starting with a letter, which would appear as blank in reports.

With 5 character numbers you're limited to 100,000 possible combinations. Just adding upper case will net 60,466,176 possible combinations. I agree with John that adding lower case, might also add to your problem.