code: @SuppressWarnings("unchecked")
      public static <T extends Number & Comparable> WindowOver<T> ntile(T num) {
          return new WindowOver<T>((Class<T>) num.getClass(), SQLOps.NTILE, ConstantImpl.create(num));
      }
docs: divides an ordered data set into a number of buckets indicated by expr and assigns the
  appropriate bucket number to each row
  
  @param num bucket size
  @return ntile(num)