查询被选最多的课程的课程名（course、selectcourse）

select c.cname from course c where c.cid in(
	-- 被选最多的课程的cid
	SELECT sc.cid from selectcourse sc GROUP BY cid
	-- 继续筛选
	having count(sid) >= all(
		select count(sid) from selectcourse sc GROUP BY cid
	)
);