code: function (columns) {
              columns = argsToArray(arguments);
              var self = this;
              return this.mergeOptions({group: (array.compact(columns).length === 0 ? null : columns.map(function (c) {
                  return isString(c) ? self.stringToIdentifier(c) : c;
              }))});
          }
docs: Returns a copy of the dataset with the results grouped by the value of
  the given columns.
  @example
  DB.from("items").group("id")
  //=>SELECT * FROM items GROUP BY id
  DB.from("items").group("id", "name")
  //=> SELECT * FROM items GROUP BY id, name
  @param {String...|patio.sql.Identifier...} columns columns to group by.
  
  @return {patio.Dataset} a cloned dataset with the GROUP BY clause added.