MySQL中的RAND()函数使用详解(order by rand() 随机查询取前几条记录)

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)

order by rand() 随机查询取前几条记录