This is a relatively small feature: SQL:2003 defines a UNIQUE predicate that takes a subquery and returns true if the subquery's result set is duplicate-free. The actual usefulness of this is debatable -- at the very least it is defined by the standard...

An implementation would want to make use of the existing machinery for performing grouping / duplicate elimination, so that this predicate can be evaluated efficiently for subqueries with arbitrarily large result sets.