Jon,

I'd like to hear if there is a way to only allow enumeration (insert a preset series of values), though this was not my intent.

I am after exactly the "primary key" restriction (insert something only if not already present), except a primary key already exists, so this is more like the UNIQUE keyword in traditional SQL.

FYI: PK is an integer and the field I'd like to have the UNIQUE constraint on is a string.

Thanks.
-Leo