MySQL特定条目数

Question

Hello i have got a Mysql table that i fill within a php script. In this script i check if the number of entries of a specific type has reached the maximum to inform the customer. If the number of entries is less i let him insert a new entry into my table. The table looks like this

--------------------
Date          | Type 
--------------------
Tuesday 13:00 | Car

And the php script that checks looks like this

<?php

$sql = $conn->prepare("SELECT count(*) FROM Table WHERE Date='Tuesday 13:00' AND Type='Car'"); 
$sql ->execute(); 
$sql_rows = $sql->fetchColumn();

if ($sql_rows > 20){ 

echo "All entries for this type has been filled please choose another to continue"; 

}else{ 

$insert = $conn->prepare("INSERT INTO Table VALUES('Tuesday 13:00', 'Car')");
$insert -> execute(); 

} 
?>

My problem now is that if 100 people access this php script at the same moment will the php script allow exactly 19 people to insert themselves 'Tuesday 13:00','Car' into the database or more and how could i prevent that without creating inconvenience to the clients?

Answer 1

Use a transaction to ensure that both queries work with the same snapshot of the table. The transaction will lock the table so that other people will wait. Put $conn->startTransaction() before the first SELECT query, add a FOR UPDATE clause to the query, and $conn->commit() after the INSERT .

You must be using InnoDB, not MyISAM, for transactions to be effective.

<?php

$sql = $conn->prepare("SELECT count(*) FROM Table 
                        WHERE Date='Tuesday 13:00' AND Type='Car' 
                        FOR UPDATE"); 
$conn->startTransaction();
$sql ->execute(); 
$sql_rows = $sql->fetchColumn();

if ($sql_rows > 20){ 
    echo "All entries for this type has been filled please choose another to continue"; 
}else{ 
    $insert = $conn->prepare("INSERT INTO Table VALUES('Tuesday 13:00', 'Car')");
    $insert -> execute(); 
} 
$conn->commit();