Question

I am currently working on a simple auction site. I am storing bids in their own MySQL table called ‘bids’. I am wondering what is the best way of ensuring that two of the same bids are not submitted at the exact same time.

My current strategy for verifying that the bid submitted is in fact the highest bid is to do the following (as an example):

$sql = "SELECT * FROM bids WHERE amount >= '".$bidamount."'";
$result = mysql_query($sql);
if(mysql_num_rows($result) == 0) {
    $sql = "INSERT INTO bids SET amount = '".$bidamount."'";
    mysql_query($sql);
    $bidid = mysql_insert_id();
}

The problem with the above set of queries is that between the time the SELECT query is run and the INSERT query is run, another user could insert the same bid.

Is there some way to lock the table during the SELECT that would prevent this double-bidding from occurring? My main concern with locking tables for such a purpose would be performance problems when you have a lot of people bidding at once.

Answer 1

You may want to make conditional insert, like:

$amount = intval($amount);
$query = "
    INSERT INTO 
        bids(amount) 
    SELECT 
        {$amount} 
    FROM 
        (SELECT 1) tmp_tbl 
    WHERE NOT EXISTS(
        SELECT * FROM bids WHERE amount >= {$amount}
    )
";

and check for affected (inserted) rows.