code: public AggregationResult aggregate(String fields) throws DDFException {
      return this.getAggregationHandler().aggregate(AggregateField.fromSqlFieldSpecs(fields));
    }
docs: Compute aggregation which is equivalent to SQL aggregation statement like
  "SELECT a, b, sum(c), max(d) FROM e GROUP BY a, b"
  
  @param fields a string includes aggregated fields and functions, e.g "a, b, sum(c), max(d)"
  @return
  @throws DDFException