Database: student_club (difficulty: simple)
Question: List the name of events with less than average Advertisement cost.
SQL: 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 = 'Advertisement' AND T3.cost < (SELECT AVG(cost) FROM expense)