Returns a random floating-point valuevin the range0<=v<1.0. If a constant integer argumentNis specified, it is used as the seed value, which produces a repeatable sequence of column values.

若未指定参数，那么返回的值为0到1之间的浮点数；若指定了随机因子那么产生的值是固定的。

测试样例如下：

mysql> select rand(),rand(5);

+-------------------+-------------------+

| rand() | rand(5) |

+-------------------+-------------------+

| 0.406958082138522 | 0.406135974830143 |

+-------------------+-------------------+

1 row in set (0.00 sec)

mysql> select rand(),rand(5);

+------------------+-------------------+

| rand() | rand(5) |

+------------------+-------------------+

| 0.16665149123096 | 0.406135974830143 |

+------------------+-------------------+

1 row in set (0.00 sec)

mysql> select rand(),rand(5);

+-------------------+-------------------+

| rand() | rand(5) |

+-------------------+-------------------+

| 0.612383240472882 | 0.406135974830143 |

+-------------------+-------------------+

1 row in set (0.00 sec)

在官方手册中，还提到如下几点：

1、不要使用rand()在查询中，因为会造成多次扫描。

You cannot use a column withRAND()values in anORDER BYclause, becauseORDER BYwould evaluate the column multiple times.

2、在SBR复制中使用rand()不安全,使用rand()会引起一个警告，官方解释如下：

Beginning with MySQL 5.1.43, this function is flagged as unsafe for statement-based replication; use of this function causes a warning when using statement-based replication, and the statement is logged using the binary format whenbinlog_formatisMIXED. (Bug #49222)