在MySQL中，想要对表的多次查询结果分别排序合并是不可行的
如：
SELECT name FROM student
ORDER BY age
UNION
SELECT grade FROM student
ORDER BY age;
为了避开语法的问题，有了下面的解决办法，使用子查询
SELECT * from
(SELECT name FROM student
ORDER BY age) t1
UNION (all)
SELECT * from
(SELECT grade FROM student
ORDER BY age);
顺利通过，可是查看结果会发现，排序失效了。因为子查询规则是没有top语句（mysql中用limit代替），
排序不生效，再次修改：
SELECT * from
(SELECT name FROM student
ORDER BY age limit 0, 1000) t1
UNION (all)
SELECT * from
(SELECT grade FROM student
ORDER BY age limit 0, 1000);
原文：https://blog.csdn.net/zhouli2008/article/details/79780117
