Among the members who incurred expenses in more than one event, who paid the most amount?
paid the most amount refers to for expense incurred in more than one event refers to MAX(cost where COUNT(event_id) > 1)
SELECT T2.member_id FROM expense AS T1 INNER JOIN member AS T2 ON T1.link_to_member = T2.member_id INNER JOIN budget AS T3 ON T1.link_to_budget = T3.budget_id INNER JOIN event AS T4 ON T3.link_to_event = T4.event_id GROUP BY T2.member_id HAVING COUNT(DISTINCT T4.event_id) > 1 ORDER BY SUM(T1.cost) DESC LIMIT 1