How to select a random record from database only once

Question

I want to create a PHP page which selects a random record from MySQL database (a string, which will be used as a unique "token"). I want to select this random record only once and never again afterwards.

In order to do so, I add an extra field to the table (named 'numberfield', containing number 0 ), which is incremented at the moment the specific record has been selected. This way, only records with the value of 0 in the numberfield table field can be selected. As soon as the random record has been selected the numberfield field is incremented to 1 and can not be selected anymore. I'm thinking about using the following PHP code:

$result = mysql_query("SELECT token FROM table WHERE numberfield < **1** ORDER BY RAND() LIMIT 0,1");

if (!$result) {
    echo 'Could not run query: ' . mysql_error();
    exit;
}

$row = mysql_fetch_row($result);
echo $row[0];
mysql_query("UPDATE table SET numberfield=numberfield+1 WHERE token=" + $row[0] + "");

Will this be the right way to do it ? Any thoughts ?

Furthermore, I want to avoid a different random record is selected at page refresh. What will be the right and most simple way to keep the "token" in cache ? For example, is it possible to keep the "token" in a session which will not be overwritten during page refresh, or do I have to use other techniques like Ajax etc. Your help and comment is highly appreciated !

Answer 1

1. You should stop using mysql_ functions as they are deprecated.
2. You should avoid using ORDER BY RAND() in larger tables because of overhead incurred from seeking a random number.
3. To accomplish a SELECT ... UPDATE you would need some sort of locking. There is a slim chance that a row could be randomly selected twice. This can be prevented using a stored procedure.
4. You can just use the session id generated by PHP or a part of it. This topic talks about how unique a session id is and this topic discusses the varying lengths.
5. You can alternatively create a random string on the PHP side which would save overhead from connecting to the database.