code: public Search groupField(String fieldName, boolean isNumber) {
          assertNotEmpty(fieldName, "fieldName");
          if (isNumber) {
              databaseHelper.query("group_field", fieldName + "<number>");
          } else {
              databaseHelper.query("group_field", fieldName);
          }
          return this;
      }
docs: Group results by the specified field.
  
  @param fieldName by which to group results
  @param isNumber  whether field isNumeric.
  @return this for additional parameter setting or to query