Question

I have a table with this columns : [user_id][game_id]
I need to close the game when two player join the game.
I used this code :

if(mysql_num_rows(mysql_query("SELECT user_id FROM live_games WHERE game_id = '$gid'"))<2){
mysql_query("INSERT INTO live_games (user_id, game_id) VALUES ('$uid', '$gid')");
echo "You have joined the game";
}else{
echo "Table is full";
}

the code will allow only two player to register but some times when there are too many users in the website , this condition will not work and three user will add in the table.
How can i fix?

Answer 1

You have a few options:

1. If using the InnoDB storage engine, perform a locking read with SELECT ... FOR UPDATE within the same transaction as the INSERT statement.

   Using PDO:

   $dbh = new PDO("mysql:dbname=$dbname;charset=utf8", $username, $password);
   $dbh->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
   
   $dbh->beginTransaction();
   $qry = $dbh->prepare('
     SELECT COUNT(*) FOR UPDATE FROM live_games WHERE game_id = ?
   ');
   $qry->execute([$gid]);
   
   if ($qry->fetchColumn() < 2) {
     $qry = $dbh->prepare('
       INSERT INTO live_games (user_id, game_id) VALUES (?, ?)
     ');
     $qry->execute([$uid, $gid]);
     if ($qry->rowCount() && $dbh->commit()) echo 'You have joined the game';
   } else {
     $dbh->rollBack();
     echo 'Table is full';
   }
   

2. Change the table structure so that there are two columns, player1 and player2 (initially NULL) and perform UPDATE live_games SET player2 = ? WHERE game_id = ? AND player2 IS NULL, then check the number of affected rows; or

3. Change the table structure so that there is an additional playerNumber column and then create a composite UNQUE index over (game_id, playerNumber).