>>>Is it possible to create an index tag on a field that would enforce unique entry in the field, unless the entry is blank (no entry)?
>>
>>You can have a candidate index On myfield with NOT EMPTY(myfield) in For clause.
>
>Thank you very much. By the way, I did try it before I posted the question but I missed entering the field name in the EMPTY() expression.

You welcome. Make sure you don't have NULL values in the field.