/**
   * Evaluates aggregate functions during the group phase.
   *
   * @param context the current database tuple
   */
  public void evalGroup(QueryContext context) throws SQLException {
    if (_expr.isNull(context)) return;

    switch (_type) {
      default:
        {
          String value = _expr.evalString(context);

          context.setGroupString(_index, value);
        }
        break;
    }
  }