如何在订购系统中预订门票？

I'm looking for advice or idea about this situation. I have a Table session. Each session can have up to X guest.

SESSION
idSession
MaxGuest

I have another table TICKETS. When someone order X tickets for a SESSION I would like to make sure there is enough ticket available between the moment he ordered the tickets and the moment the system has recorded his order.

Example:

If I have many orders at the same time I may have this problem.

The only idea I see is to have a table to store current process order and check the qty available from the number of tickets already ordered and the number of ticket being ordered.

Does anyone has a better idea? I wonder if it's not too "heavy"? Any help would be appreciated.

You need to use AJAX and PHP for this idea to work. Without AJAX, information could be retrieved from the server that is not recent. Orders can happen in seconds. I would do something like this:

init.php

if($watching){
    echo 1;
} else {
    echo 0;
}

You could set $watching to retrieve information from your db and check if items are still available.

page.php

<script>
    function isWatching() {
        $.get('init.php', function(data){
                if(data == 1) {
                    alert('You are watching a video');
                } else if(data == 0) {
                    alert('You are not watching a video');
                }
        });
    }
    setTimeout(isWatching, 1000);
</script>

NOTE: I would not recommend setting the interval to one second.