phpmysql

MySQL get a random value between two values


I have two columns in a row: min_value, max_value. Is there a way to do a select like:

SELECT RAND(`min_v`, `max_v`) `foo` [..]

I do realize that RAND does a different thing; the closest I came up (with help) is (RAND() * (max-min))+min, though it will produce a float number, which I'd need then to ROUND() and this is just completely wrong.

Unless anyone can suggest an alternative (which would be very useful), I will go PHP way.


Solution

  • Actually, ROUND((RAND() * (max-min))+min) is the best way in MySQL to do what you'd like. It is also the best way in ActionScript, JavaScript, and Python. Honestly, I prefer it to the PHP way because it is more convenient.

    Because I don't know how many rows you'll be returning, I can't advise you whether it is better to use PHP or MySQL for this, but if you're dealing with a large number of values you probably are better off using MySQL.

    Addendum


    So, there was a question as to whether this is better in PHP or MySQL. Instead of getting into a debate on principles, I ran the following:

    <pre><?php
    
    $c = mysql_connect('localhost', 'root', '');
    
    if(!$c) die('!');
    echo mysql_select_db('test', $c)?'Connection':'Failure';
    echo PHP_EOL;
    
    echo ':::::::::::::::::::::::::BEGINNING MYSQL RAND::::::::::::::::::::::::::::::'.PHP_EOL;
    $start = microtime(1);
    for( $i = 0; $i < 100000; $i++ )
    {
        $r = mysql_query( 'SELECT ROUND(RAND() * (200-10) + 10) FROM dual' );
        $r = mysql_fetch_array( $r );
    }
    $end = microtime(1);
    
    echo  ($end - $start) . " for MySQL select".PHP_EOL;
    
    echo ':::::::::::::::::::::::::BEGINNING PHP RAND::::::::::::::::::::::::::::::' .PHP_EOL;
    $start = microtime(1);
    for( $i = 0; $i < 100000; $i++ )
    {
        $r = mysql_query( 'SELECT 200 AS two, 10 AS tem FROM dual' );
        $r = mysql_fetch_array( $r );
        $r[2]= rand($r[0], $r[1]);
    }
    $end = microtime(1);
    
    echo  ($end - $start) . " for PHP select".PHP_EOL;
    

    MySQL is faster by about 2-3%.

    If you use this, however (note, more columns return by MySQL):

    <pre><?php
    
    $c = mysql_connect('localhost', 'root', '');
    
    if(!$c) die('!');
    echo mysql_select_db('test', $c)?'Connection':'Failure';
    echo PHP_EOL;
    
    echo ':::::::::::::::::::::::::BEGINNING MYSQL RAND::::::::::::::::::::::::::::::'.PHP_EOL;
    $start = microtime(1);
    for( $i = 0; $i < 100000; $i++ )
    {
        $r = mysql_query( 'SELECT ROUND(RAND() * (200-10) + 10) as rd, 200 as two, 10 as ten FROM dual' );
        $r = mysql_fetch_array( $r );
    }
    $end = microtime(1);
    
    echo  ($end - $start) . " for MySQL select".PHP_EOL;
    
    echo ':::::::::::::::::::::::::BEGINNING PHP RAND::::::::::::::::::::::::::::::' .PHP_EOL;
    $start = microtime(1);
    for( $i = 0; $i < 100000; $i++ )
    {
        $r = mysql_query( 'SELECT 200 AS two, 10 AS tem FROM dual' );
        $r = mysql_fetch_array( $r );
        $r[2]= rand($r[0], $r[1]);
    }
    $end = microtime(1);
    
    echo  ($end - $start) . " for PHP select".PHP_EOL;
    

    MySQL comes out behind by 3-4% (very inconsistent results) (about the same results if you don't use an array index assignment for $r[2]).

    The major difference, it seems, comes from the number of records return to PHP and not the randomization system itself. So, if you need column A, column B, and a random value, use PHP. If you only need the random value, then use MySQL.