Question

I need to do two updates to rows but I need to make sure they are done together and that no other query from another user could interfere with them. I know about SELECT...FOR UPDATE but I imagine after the first update it will of course be unlocked which means someone could interfere with the second update. If someone else updates that row first, the update will work but will mess up the data. Is there anyway to ensure that the two updates happen how they are supposed to? I have been told about transactions but as far as I know they are only good for making sure the two updates actually happen and not whether they happen “together,” unless I am mistaken and the rows will be locked until the transaction is committed?

Here are the queries:

SELECT z FROM table WHERE id='$id'

UPDATE table SET x=x+2 WHERE x>z

UPDATE table SET y=y+2 WHERE y>z

I made a mistake and didn’t give full information. That was my fault. I have updated the queries. The issue I have is that z can be updated as well. If z is updated after the SELECT but before the other two updates, the data can get messed up. Does doing the transaction BEGIN/COMMIT work for that?

Answer 1

Learn about TRANSACTION
http://dev.mysql.com/doc/refman/5.0/en/commit.html

[... connect ...]

mysql_query("BEGIN");

$query1 = mysql_query('UPDATE table SET x=x+2 WHERE x>y');
$query2 = mysql_query('UPDATE table SET y=y+2 WHERE y>y');

if($query1 && $query2) {
    mysql_query("COMMIT");  
    echo 'Save Done. All UPDATES done.';  
} else {
    mysql_query("ROLLBACK");  
    echo 'Error Save. All UPDATES reverted, and not done.';  
}