有同学上周问了个问题 “MySQL 里面的order by rand()”是怎么实现的。我们今天来简单说说MySQL里的order by。          几种order by的情况         乍一看这个问题好像有点复杂，我们从最简单的case开始看起。

    mysql> use test;Database changedmysql> select count(*) from test;+----------+| count(*) |+----------+| 2097152 |+----------+1 row in set (0.00 sec)mysql> select * from test ORDER BY RAND() LIMIT 1;+--------+------+| a | b |+--------+------+| 765909 | |+--------+------+1 row in set (8.78 sec) --用了将近9s 引Mysql文档：You cannot use a column with RAND() values in an ORDER BY ...