Unique Values in a field

No there isn't.

You have to create a temptable with all the unique values you encounter in a field.

May be you can try sorting and grouping with a report on that field?

This is also a possibility, but keep in mind that this takes away extra DB-space (not really a problem) and also costs writing-performance (this can be a problem).
Before creating a new index in a table (if an index starting with the field you need, you can use it), you have to consider a few things.
-If it is a table where a lot of writes happen (specially the entry-tables), it is better to avoid an extra index.
-If your report is not to urgent (meaning when the user starts it, he NEEDS it in 2 seconds), you may consider not to create a new index.
-if you have a table that isn't changed a lot, and you have lots of records (at least 50000),you might consider creating the index.