PDO exception on correct MySql query - php

Question

I'm trying to update a MySql database using PDO in PHP.
I need to launch 4 lines of SQL code. The last one is the SELECT statement, which should return only one integer value.
The error thrown is:
exception 'PDOException' with message 'SQLSTATE[HY000]: General error' in C:\xampp\htdocs\php\set-lesson-finished.php:22
Stack trace:
#0 C:\xampp\htdocs\php\set-lesson-finished.php(22): PDOStatement->fetchColumn()
#1 {main}
And here is the code:
try {
$db_connection = new PDO('mysql:host='. DB_HOST .';dbname='. DB_NAME . ';charset=utf8', DB_USER, DB_PASS, array(
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));
$query_save = $db_connection->prepare('
DELETE FROM unfinished_lessons WHERE bought_id = :bought_id;
INSERT INTO finished_lessons VALUES (:bought_id, :time_invested, NOW());
UPDATE users SET points = points + :points_earned WHERE id = :user_id;
SELECT points FROM users WHERE id = :user_id;
');
$query_save->bindValue(':user_id', (int)$_SESSION['user_id'], PDO::PARAM_INT);
$query_save->bindValue(':bought_id', (int)$_POST['bought_id'], PDO::PARAM_INT);
$query_save->bindValue(':time_invested', (int)$_POST['time_invested'], PDO::PARAM_INT);
$query_save->bindValue(':points_earned', (int)$_POST['points_earned'], PDO::PARAM_INT);
$query_save->execute();
echo $query_save->fetchColumn();
} catch (PDOException $e) {
echo $e;
}

Answer 1

This is not a single query but four queries together. You'll get four different result sets which need to be retrieved independently.
The error you're seeing is occurring because you can't call fetchColumn() on a DELETE query (it doesn't return a result set). Likewise for INSERT and UPDATE.
You could experiment with PDOStatement::nextRowSet(), but it's probably easier to run the four queries separately. If you're concerned about having an all-or-nothing update you should use a transaction with PDO::beginTransaction() and PDO::commit()

Answer 2

When queries gets complex you need to use a stored procedure:
DELIMITER //
CREATE PROCEDURE your_sp(IN `p_bought_id`, IN `p_time_invested`, IN `p_points_earned`, IN `p_user_id`)
BEGIN
DELETE FROM unfinished_lessons WHERE bought_id = p_bought_id;
INSERT INTO finished_lessons VALUES (p_bought_id, p_time_invested, NOW());
UPDATE users SET points = points + p_points_earned WHERE id = p_user_id;
SELECT points FROM users WHERE id = p_user_id;
END //
DELIMITER ;
You call it in one query like this:
$query_save = $db_connection->prepare('
call your_sp(:bought_id, :time_invested,:points_earned, :user_id)
');

Answer 3

You should use mysqli extension (and not PDO) if you absolutely want to run multiple statements at the same time.
As you can see in this Feature comparison (see the comparison table), only ext/mysqli has full support for multiple MySQL queries (and also supports all MySQL 5.1+ functionnality).
And, as Drupal's dawehner says, MySQL PDO no longer allows multiple database queries to be executed at the same time.

Answer 4

I've split the queries in four and everything works as expected:
$db_connection = new PDO('mysql:host='. DB_HOST .';dbname='. DB_NAME . ';charset=utf8', DB_USER, DB_PASS, array(
PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION));
$db_connection->beginTransaction();
$query = $db_connection->prepare('DELETE FROM planz_unfinished_lessons WHERE bought_id = :bought_id;');
$query->bindValue(':bought_id', (int)$_POST['bought_id'], PDO::PARAM_INT);
$query->execute();
$query = $db_connection->prepare('INSERT INTO planz_finished_lessons VALUES (:bought_id, :time_invested, NOW())');
$query->bindValue(':bought_id', (int)$_POST['bought_id'], PDO::PARAM_INT);
$query->bindValue(':time_invested', (int)$_POST['time_invested'], PDO::PARAM_INT);
$query->execute();
$query = $db_connection->prepare('UPDATE planz_users SET points = points + :points_earned WHERE id = :user_id');
$query->bindValue(':points_earned', (int)$_POST['points_earned'], PDO::PARAM_INT);
$query->bindValue(':user_id', (int)$_SESSION['user_id'], PDO::PARAM_INT);
$query->execute();
$query_check = $db_connection->prepare('SELECT points FROM planz_users WHERE id = :user_id');
$query_check->bindValue(':user_id', (int)$_SESSION['user_id'], PDO::PARAM_INT);
$query_check->execute();
$db_connection->commit();
$_SESSION['points'] = $query_check->fetchColumn();
echo $_SESSION['points'];
Note the use of beginTransaction and commit, as they should speed up the querying.