@staticmethod
def _get_group_by_period_list(aggregation_period):
    """get a list of columns for an aggregation period."""
    group_by_period_list = []
    if aggregation_period == 'daily':
        group_by_period_list = ['event_date']
    elif aggregation_period == 'hourly':
        group_by_period_list = ['event_date', 'event_hour']
    elif aggregation_period == 'minutely':
        group_by_period_list = ['event_date', 'event_hour', 'event_minute']
    elif aggregation_period == 'secondly':
        group_by_period_list = ['event_date', 'event_hour', 'event_minute',
            'event_second']
    return group_by_period_list
