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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

select c.cname,c.ctimenumber from course c where c.cid not in ( select cid from selectcourse sc); select c.cname,c.ctimenumber from course c left outer join selectcourse sc on c.cid = sc.cid whe

select ce.cname,count(se.cid) from course ce LEFT JOIN selectcourse se on se.cid = ce.cid group by ce.cid;count（*）将nulll同样计算在其中count(指定) 可以去除null值