MySQL Cookbook

13.7 Generating Random Numbers

13.7.1 Problem

You need a source of random numbers.

13.7.2 Solution

Invoke MySQL's RAND( ) function.

13.7.3 Discussion

MySQL has a RAND( ) function that can be invoked to produce random numbers between 0 and 1:

mysql> SELECT RAND( ), RAND( ), RAND( );
+------------------+------------------+------------------+
| RAND( )           | RAND( )           | RAND( )           |
+------------------+------------------+------------------+
| 0.31466114177803 | 0.89354679723601 | 0.52375059157959 |
+------------------+------------------+------------------+

When invoked with an integer argument, RAND( ) uses that value to seed the random number generator. Each time you seed the generator with a given value, RAND( ) will produce a repeatable series of numbers:

mysql> SELECT RAND(1), RAND( ), RAND( );
+------------------+------------------+------------------+
| RAND(1)          | RAND( )           | RAND( )           |
+------------------+------------------+------------------+
| 0.18109050223705 | 0.75023211143001 | 0.20788908117254 |
+------------------+------------------+------------------+
mysql> SELECT RAND(20000000), RAND( ), RAND( );
+------------------+-------------------+------------------+
| RAND(20000000)   | RAND( )            | RAND( )           |
+------------------+-------------------+------------------+
| 0.24628307879556 | 0.020315642487552 | 0.36272900678472 |
+------------------+-------------------+------------------+
mysql> SELECT RAND(1), RAND( ), RAND( );
+------------------+------------------+------------------+
| RAND(1)          | RAND( )           | RAND( )           |
+------------------+------------------+------------------+
| 0.18109050223705 | 0.75023211143001 | 0.20788908117254 |
+------------------+------------------+------------------+
mysql> SELECT RAND(20000000), RAND( ), RAND( );
+------------------+-------------------+------------------+
| RAND(20000000)   | RAND( )            | RAND( )           |
+------------------+-------------------+------------------+
| 0.24628307879556 | 0.020315642487552 | 0.36272900678472 |
+------------------+-------------------+------------------+

If you want to seed RAND( ) randomly, pick a seed value based on a source of entropy. Possible sources are the current timestamp or connection identifier, alone or perhaps in combination:

mysql> SELECT RAND(UNIX_TIMESTAMP( )) AS rand1,
    -> RAND(CONNECTION_ID( )) AS rand2,
    -> RAND(UNIX_TIMESTAMP( )+CONNECTION_ID( )) AS rand3;
+------------------+------------------+------------------+
| rand1            | rand2            | rand3            |
+------------------+------------------+------------------+
| 0.50452774158169 | 0.18113064782799 | 0.50456789089792 |
+------------------+------------------+------------------+

However, it's probably better to use other seed value sources if you have them. For example, if your system has a /dev/random or /dev/urandom device, you can read the device and use it to generate a value for seeding RAND( ).