社区活动中心需要查找每个活动的参与人数，表结构如下： Eve

SELECT event_name, SUM(participant_id)  FROM Events JOIN Participants ON Events.event_id = Participants.event_id;

SELECT event_name FROM Events WHERE event_id IN   (SELECT event_id FROM Participants);

SELECT event_id, COUNT(participant_id) FROM Participants GROUP BY event_id;

SELECT event_name, COUNT(*) FROM Events JOIN Participants ON Events.event_id = Participants.event_id GROUP BY event_name;