select t.Topic, t.Title, count(distinct s.starID) as StarCount, count(distinct m.User) as UserCount, count(distinct m.messageID) as MessageCountfrom Topics t left join Messages m ON m.Topic = t.Topic left join Stars_Given s ON s.Topic = t.Topicgroup by t.Topic, t.Title
SQL小提琴
或者，您可以在子查询中执行聚合，如果表中有大量数据，这可能会更有效：
select t.Topic, t.Title, s.StarCount, m.UserCount, m.MessageCountfrom Topics t left join ( select Topic, count(distinct User) as UserCount, count(*) as MessageCount from Messages group by Topic ) m ON m.Topic = t.Topic left join ( select Topic, count(*) as StarCount from Stars_Given group by Topic ) s ON s.Topic = t.Topic
