List the name of events with less than average parking cost.
name of events refers to event_name; less than average parking cost refers to cost < DIVIDE(SUM(cost), COUNT(event_id)) where category = 'Parking'
SELECT T1.event_name FROM event AS T1 INNER JOIN budget AS T2 ON T1.event_id = T2.link_to_event INNER JOIN expense AS T3 ON T2.budget_id = T3.link_to_budget WHERE T2.category = 'Parking' AND T3.cost < (SELECT AVG(cost) FROM expense)