SQL RAND() 函数

SQL RAND 函数

SQL 有一个 RAND 函数，用于产生 0 至 1 之间的随机数：

SQL>  SELECT RAND( ), RAND( ), RAND( );
+------------------+-----------------+------------------+
| RAND( )          | RAND( )         | RAND( )          |
+------------------+-----------------+------------------+
| 0.45464584925645 | 0.1824410643265 | 0.54826780459682 |
+------------------+-----------------+------------------+
1 row in set (0.00 sec)

当以某个整数值作为参数来调用的时候，RAND() 会将该值作为随机数发生器的种子。对于每一个给定的种子，RAND() 函数都会产生一列可以复现的数字：

SQL>  SELECT RAND(1), RAND( ), RAND( );
+------------------+------------------+------------------+
| RAND(1 )         | RAND( )          | RAND( )          |
+------------------+------------------+------------------+
| 0.18109050223705 | 0.75023211143001 | 0.20788908117254 |
+------------------+------------------+------------------+
1 row in set (0.00 sec)

你可以使用 ORDER BY RAND() 来对一组记录进行随机化排列，如下所示：

SQL> 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，试试下面的命令：

SQL> 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)

SQL> 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)