Busy PHP connection to MYSQL?

Question

This is a part of PHP & MySQL Novice to Ninja, 5th Edition by Kevin Yank

Normally when we perform a SELECT query , we use the condition of a while loop or a foreach loop to retrieve each row of the results, one at a time:

while ($row = $result->fetch()) foreach ($result as $row)

When we process the result of a query this way, PHP actually retrieves each row from the database as the loop requests it, and throws it away when it moves on to the next row . This saves PHP from having to use a lot of memory to hold onto all the rows of results at once.

Most of the time, we developers have no need to know that PHP is taking this clever shortcut. But every once in a while, we’ll want to send another SQL query to the MySQL server before we’ve worked through all the results of the previous query .

That’ s exactly what’ s about to happen in this code if we aren’t careful: we’ve just run a SELECT query to ask for a list of all jokes belonging to a particular author, and as we work through that list, we’d like to perform a DELETE query for each one. The problem is, as far as MySQL knows, it will still be busy sending us the results of the SELECT query; we can’t just interrupt it and ask for it to start running DELETEs! Doing so would cause our DELETEs to fail with an error .

That’ s where the fetchAll method comes in. By calling this method on our prepared statement ($s), we ask PHP to retrieve the entire set of results for the query and store them in a PHP array ($result):

My question is Why I can execute MYSQL statements while I have one or more PDOStatement result sets being used in my PHP code? even I can delete all rows being referenced in those result sets

<?php
    include_once $_SERVER["DOCUMENT_ROOT"] . '/DBconnection.inc.php';

        $pds1 = $dbi->query("SELECT name FROM tbl1");
        $row = $pds1->fetch();
        var_dump($row['name']);

        $pds2 =  $dbi->query("SELECT user FROM tbl2");
        $row = $pds2->fetch();
        var_dump($row['user']);

        $row = $pds1->fetch();
        var_dump($row['name']);

        $row = $pds2->fetch();
        var_dump($row['user']);

        $pds3= $dbi->prepare("DELETE FROM tbl1;");
        $pds3->execute();

        $row = $pds1->fetch();
        var_dump($row['name']);

        $row = $pds2->fetch();
        var_dump($row['user']);



?>

Answer 1

This is an area where the behavior of PDO is going to depend on the database you're using and the PDO driver implementation. You might be able to execute new queries while you're still fetching the results of a select statement, or you might not.

In general, the safest and most compatible way to use PDO connections is:

1. execute your SELECT statement
2. fetch all of the rows you need
3. close the cursor (eg: with $pds1->closeCursor())
4. execute your next query