What are the categories of the top 2 oldest events?
oldest event refers to MIN(timestamp);
SELECT T4.category FROM events_relevant AS T1 INNER JOIN app_events_relevant AS T2 ON T1.event_id = T2.event_id INNER JOIN app_labels AS T3 ON T3.app_id = T2.app_id INNER JOIN label_categories AS T4 ON T3.label_id = T4.label_id ORDER BY T1.timestamp LIMIT 2