SQL语句
描述：exhibitiononline表主键exhiid， exhibitor表有外键 exhid，对应表 exhibitiononline的主键exhiid；
exhibitiononline表---------> 一对多 <------------- exhibitor表
功能：查询出 exhibitiononline表中，exhiid存在于表 exhibitor中的所有数据，并根据 exhibitor表的exhid的数量排序
select * from exhibitiononline join
(select exhid,count(exhid) as s from exhibitor group by exhid) as t
on exhibitiononline.exhiid=t.exhid
order by t.s desc