code: @NonNull
      @Override
      public GroupBy groupBy(@NonNull Expression... expressions) {
          if (expressions == null) {
              throw new IllegalArgumentException("expressions cannot be null.");
          }
          return new GroupBy(this, Arrays.asList(expressions));
      }
docs: Creates and chains a GroupBy object to group the query result.
  
  @param expressions The group by expression.
  @return The GroupBy object that represents the GROUP BY clause of the query.