PHP MySQL Weird Update Problem

Question

I have a heap based table in MySQL that I am trying to update via PHP, but for some reason, the updates do not seem to be taking place.

Here is my test code:

<?php
$freepoints[] = 1;
$freepoints[] = 2;
$freepoints[] = 3;
foreach ($freepoints as $entrypoint) {
$query = "update gates set lane='{$entrypoint}' where traffic > 50 limit 50";
echo "$query\n";
mysql_query($query);
echo mysql_affected_rows()."\n";
}
?>

This outputs the following:

update gates set lane='1' where traffic > 50 limit 50
50
update gates set lane='2' where traffic > 50 limit 50
50
update gates set lane='3' where traffic > 50 limit 50
50

In the database to start with lanes 1/2/3 had 0 records and lanes 4/5/6 had 100 records. From this I am expecting all 6 lanes to now have 50 records each. However when I look lanes 4/5/6 still have 100 records and 1/2/3 still have 0 records.

When I copy the query "update gates set lane='1' where traffic > 50 limit 50" into phpMyAdmin it works absolutely fine, so any ideas why it isn't working in my PHP script when mysql_affected_rows is saying it has updated 50 records?

Answer 1

1. Add ORDER BY clause
2. you are always rewriting with the same data. you need to specify WHERE with lane to update another rows