Subqueries in basic predicates cannot use GROUP BY

  Explanation:  A subquery used in a basic predicate, such as
  WHERE CITY = (SELECT ...), cannot contain the GROUP BY clause.
  The ANSI standard does not support this.

  User Action:  If it is important to have only standard SQL
  statements in your program, remove the GROUP BY clause or change
  the predicate to include ANY or ALL.