MySQL中的RAND()函数使用详解

mysql rand()函数调用可以在0和1之间产生一个随机数：

mysql> select rand( ), rand( ), rand( );
+------------------+-----------------+------------------+
| rand( )   | rand( )   | rand( )   |
+------------------+-----------------+------------------+
| 0.45464584925645 | 0.1824410643265 | 0.54826780459682 |
+------------------+-----------------+------------------+
1 row in set (0.00 sec)

当调用一个整数参数时，rand()使用该随机数发生器种子值。每次在给定值种子生成，rand()会产生一个可重复的一系列数字：

mysql> select rand(1), rand( ), rand( );
+------------------+------------------+------------------+
| rand(1 )   | rand( )   | rand( )   |
+------------------+------------------+------------------+
| 0.18109050223705 | 0.75023211143001 | 0.20788908117254 |
+------------------+------------------+------------------+
1 row in set (0.00 sec)

可以使用order by rand()，随机组行或值如下：

要了解order by rand()函数, 假设employee_tbl的表有以下记录：

mysql> select * from employee_tbl;
+------+------+------------+--------------------+
| id | name | work_date | daily_typing_pages |
+------+------+------------+--------------------+
| 1 | john | 2007-01-24 |    250 |
| 2 | ram | 2007-05-27 |    220 |
| 3 | jack | 2007-05-06 |    170 |
| 3 | jack | 2007-04-06 |    100 |
| 4 | jill | 2007-04-06 |    220 |
| 5 | zara | 2007-06-06 |    300 |
| 5 | zara | 2007-02-06 |    350 |
+------+------+------------+--------------------+
7 rows in set (0.00 sec)

现在使用以下目录：

mysql> select * from employee_tbl order by rand();
+------+------+------------+--------------------+
| id | name | work_date | daily_typing_pages |
+------+------+------------+--------------------+
| 5 | zara | 2007-06-06 |    300 |
| 3 | jack | 2007-04-06 |    100 |
| 3 | jack | 2007-05-06 |    170 |
| 2 | ram | 2007-05-27 |    220 |
| 4 | jill | 2007-04-06 |    220 |
| 5 | zara | 2007-02-06 |    350 |
| 1 | john | 2007-01-24 |    250 |
+------+------+------------+--------------------+
7 rows in set (0.01 sec)

mysql> select * from employee_tbl order by rand();
+------+------+------------+--------------------+
| id | name | work_date | daily_typing_pages |
+------+------+------------+--------------------+
| 5 | zara | 2007-02-06 |    350 |
| 2 | ram | 2007-05-27 |    220 |
| 3 | jack | 2007-04-06 |    100 |
| 1 | john | 2007-01-24 |    250 |
| 4 | jill | 2007-04-06 |    220 |
| 3 | jack | 2007-05-06 |    170 |
| 5 | zara | 2007-06-06 |    300 |
+------+------+------------+--------------------+
7 rows in set (0.00 sec)