关于mysql 的order by rand()

 官方手册上是这么写的：
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You cannot
use a column with RAND()
values in an ORDER BY clause, because ORDER BY would evaluate the column
multiple times. However, you can retrieve rows in random order like this:
mysql> SELECT
* from tbl_name
ORDER BY RAND();
ORDER BY
RAND() combined with LIMIT is useful for selecting a random sample from a set
of rows:
mysql> SELECT
* from table1, table2 WHERE a=b AND c<d
-> ORDER BY RAND() LIMIT 1000;
RAND()
is not meant to be a perfect random generator, but instead is a fast way to
generate ad hoc
random numbers which is portable between platforms for
the same MySQL ver