对于union的简单介绍可以参考这里 。 如果你需要对各自的结果集先进行排序，然后在union起来的话： 一 首先举出错误的做法（代码为php） $sql1 = “select * from table where num>100 order by timestamp desc”; $sql2 = “select * from table where num<=100 order by timestamp asc”;
如果将 $sql1 union $sql2 来执行的话，最后结果一般是乱序。无规律可循。
二 正确做法:
$sql1 = “select * from table where num>100 order by timestamp desc”; $sql2 = “select * from table where num<=100 order by timestamp asc”; $SQL= “select * from ($sql1) as temp1 union select * from ($sql2) as temp2″;
