关于表连接如(union join)中使用order limit
Eg: select 1 order by 1 union select 2 order by 1
正确使用方法
（select 1 order by 1 limit 1） union (select 2 order by 2) order by 1
对各个结果集排序
Incorrect usage of UNION and ORDER BY
原文：http://www.cnblogs.com/hackforfun/p/4183350.html