Combine SELECT and UPDATE to avoid duplicate selects - php

Question

i want to combine a SELECT and UPDATE query, to avoid duplicat select of rows.
Here is my example code:
private function getNewRCode() {
$getrcodesql = "SELECT * FROM `{$this->mysqlprefix}codes` WHERE `used` = 0 LIMIT 1;";
$getrcodequery = $this->mysqlconn->query($getrcodesql);
if(#$getrcodequery->num_rows > 0){
$rcode = $getrcodequery->fetch_array();
$updatercodesql = "UPDATE `{$this->mysqlprefix}codes` SET `used` = '1' WHERE `id` = {$rcode['id']};";
$this->mysqlconn->query($updatercodesql);
$updateusersql = "UPDATE `{$this->mysqlprefix}users` SET `used_codes` = `used_codes`+1, `last_code` = '{$rcode['code']}', `last_code_date` = NOW() WHERE `uid` = {$this->uid};";
$this->mysqlconn->query($updateusersql);
$output = array('code' => $rcode['code'],
'time' => time() + 60*60*$this->houroffset,
'now' => time()
);
return $output;
}
}
I would like to execute $getrcodesql and $updatercodesql at once, to avoid that the same code is used for different users.
I hope you understand my problem and know a solution for this.
Greetings,
Frederick

Answer 1

It's easier if you do it the other way round.
The point is that that your client can generate a unique value before you do the UPDATE and SELECT.
Change the type of your used column to something else, so that you can store a GUID or a timestamp in it, and not just 0 and 1.
(I'm not a PHP/MySQL expert, so you probably know better than me what exactly to use)
Then you can do this (in pseudocode):
// create unique GUID (I don't know how to do this in PHP, but you probably do)
$guid = Create_Guid_In_PHP();
// update one row and set the GUID that you just created
update codes
set used = '$guid'
where id in
(
select id
from codes
where used = ''
limit 1
);
// now you can be sure that no one else selected the row with "your" GUID
select *
from codes
where used = '$guid'
// do your stuff with the selected row