How to remove previous sql insert if one after fails? - php

Question

When I insert a new record into one table (work_log), I update a record in another table (employers) with the last inserted record from work_log.
But if updating employers-table doesn't succeed, after successfully inserted the new record into work_log, I need to remove the newly added record to work_log since that entry would not be valid anymore.
Here's my script so far:
/**
* This first part has no direct affect on the question, but serves as additional information to understand the script better..
* - - -
* First, a new work session is inserted (this session has nothing to do with browser session)
* If this fails, the script does not continue, and the user is redirected back to the form with an error-message.
* otherwise, the script continues, and try to activate the session by adding a new work_log entry.
*/
$ins_session = $con['site']->prepare('INSERT INTO work_sessions (fields) VALUES (?)');
$ins_session->execute(array(values));
if($ins_session){
KD::notice('success','New work session is created.');
$session_id = $con['site']->lastInsertId();
} else {
KD::notice('error','Work session was not created.');
KD::redirect(); // stops the script, and redirects
}
/**
* This part affects my question
* - - -
* Add a new entry to the work log in order to automatically start the work session.
* If this entry is successfully inserted, then add an indicator to the corresponding employer, in the employers table, to indicate that this employer has an active session (and which one it is).
*/
$ins_work_log = $con['site']->prepare('INSERT INTO work_log (fields) VALUES (?)');
$ins_work_log->execute(array(values));
if($ins_work_log){
$upd_employer = $con['site']->prepare('UPDATE employers SET fk_work_sessions_id = ? WHERE id = ?');
$upd_employer->execute(array($session_id,$_POST['employer_id']));
if($upd_employer){
KD::notice('success','New session was created and started.');
KD::redirect();
} else {
// need to remove the entry from work_log.
KD::notice('Work session was created, but not started. Please start the session manually.');
}
}
To my understanding, I have to delete the last inserted record in the work_log-table?
Is there any other way to do this? like, in another order, or to automatically remove the entry from work_log if this (the update query) fails?
The work_log-table is innoDB, and row format is compact if that is important to know...
UPDATE
I've set it up like this:
It seems to work, but I'm a bit unsure if I'm using it correctly regarding the if/else statements.
$con['site']->beginTransaction();
$ins_work_log = $con['site']->prepare('INSERT INTO work_log (fields) VALUES (?)');
$ins_work_log->execute(array(values));
if($ins_work_log){
# update employer
$upd_employer = $con['site']->prepare('UPDATE employers SET fk_work_sessions_id = ? WHERE id = ?');
$upd_employer->execute(array($session_id,$_POST['employer_id']));
if($upd_employer){
$con['site']->commit();
KD::notice('success','New session was created and started.');
} else {
$con['site']->rollBack();
KD::notice('error','Work session was created, but not started. Please start the session manually.');
}
//
} else {
$con['site']->rollBack();
KD::notice('error','');
}
KD::redirect();
Will if($ins_work_log), and if($upd_employer), have any affect when the query hasn't been committed yet?

Answer 1

This is a classic case for using START TRANSACTION, COMMIT, and ROLLBACK.
http://dev.mysql.com/doc/refman/5.0/en/commit.html
Just make sure you are using a database engine that supports it.
Pseudocode:
query("START TRANSACTION;");
query("INSERT INTO table1 ...");
if (query("INSERT INTO table2 ..."))
query("COMMIT;");
else
query("ROLLBACK;");