Take this:
and make a comma separated list od the id's which you want to update
[php]
$sql = 'SELECT id FROM `phplist_user_user_attribute` WHERE `attributeid`= 7 AND `value`= "68104"';
$query = mysql_query($sql);
if mysql_num_rows($query) {
$ary = implode(',', mysql_fetch_assoc($query, MYSQL_NUM));
$update = "UPDATE `phplist_user_user_attribute` SET `value` = '' WHERE `attributeid` ='16' AND `userid` IN (".$ary.")";
mysql_query($update);
}
[/php]
hope this helps.