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( ).

Using the mysql Client Program

Writing MySQL-Based Programs

Record Selection Techniques

Working with Strings

Working with Dates and Times

Sorting Query Results

Generating Summaries

Modifying Tables with ALTER TABLE

Obtaining and Using Metadata

Importing and Exporting Data

Generating and Using Sequences

Using Multiple Tables

Statistical Techniques

Handling Duplicates

Performing Transactions

Introduction to MySQL on the Web

Incorporating Query Resultsinto Web Pages

Processing Web Input with MySQL

Using MySQL-Based Web Session Management

Appendix A. Obtaining MySQL Software

Appendix B. JSP and Tomcat Primer

Appendix C. References



MySQL Cookbook
MySQL Cookbook
ISBN: 059652708X
EAN: 2147483647
Year: 2005
Pages: 412
Authors: Paul DuBois

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