ORDER BY 后可加2个字段，用英文逗号隔开。
f1用升序， f2降序，sql该这样写
ORDER BY f1, f2 DESC
也可以这样写，更清楚：
ORDER BY f1 ASC, f2 DESC
如果都用降序，必须用两个desc
ORDER BY f1 DESC, f2 DESC
注:
GROUP BY a,b having (a>10) order by c,d
