ON DUPLICATE KEY query - pulling out the primary key of a table - php

Question

I have a query which I would like to INSERT some data into my table however, if there is already data inside of a particular field checkPoint, then run an UPDATE. After a lot of research on here, users have suggested using ON DUPLICATE KEY.
This query works however as apposed to updating an already existing row, it inserts a new one, with a new primary key, please can someone explain where I have gone wrong, or what I've missed out.
<?php
$idUsers = $_SESSION['id'];
$ModuleID = 5;
$checkPoint = 999;
$query= "INSERT INTO `userTakingModule` (`userTakingModuleID`, `idUsers`, `ModuleID`, `checkPoint`) VALUES (NULL, $idUsers, $ModuleID, $checkPoint) ON DUPLICATE KEY UPDATE `idUsers` = VALUES ($idUsers), `ModuleID` = VALUES ($ModuleID), `checkPoint` = VALUES ($checkPoint) ";
$result = $conn -> query($query);
?>
Screenshot of my database layout: the table called userTakingModule in the middle is where the query is applied to.
This is what is happening at the moment as I need to include the Primary Key of userTakingModuleID into the query somehow. (I almost need to say, look for where there is an already existing entry of the same idUser and ModuleID?)

Answer 1

The important part of using INSERT...ON DUPLICATE KEY UPDATE is telling MySQL what the key is, so it can look for duplicates. You said:
I almost need to say, look for where there is an already existing entry of the same idUser and ModuleID
And that's exactly right. You need to create a UNIQUE index on those two columns like so:
ALTER TABLE userTakingModule ADD UNIQUE INDEX(idUser, ModuleID);
Now, conflicts will trigger the update functionality. You should just remove the userTakingModuleID column from your query altogether, it will be given a value automatically as needed. You're also mis-using the VALUES function; you should pass it a column name, and it will resolve to the value that would have been inserted into that column without a conflict. So you can use either the VALUES function, or the variable itself.
And speaking of variables, I would be remiss if I didn't point out how insecure and dangerous it is to insert variables directly into queries. You should always use prepared statements. You don't provide enough code to know which database API you're using, but for PDO it would look like this:
$idUsers = $_SESSION['id'];
$ModuleID = 5;
$checkPoint = 999;
$query= "INSERT INTO userTakingModule (idUsers, ModuleID, checkPoint) VALUES (?, ?, ?) ON DUPLICATE KEY UPDATE idUsers = VALUES (idUsers), ModuleID = VALUES (ModuleID), checkPoint = VALUES (checkPoint)";
$stmt = $conn->prepare($query);
$stmt->execute([$idUsers, $ModuleID, $checkPoint]);
$data = $stmt->fetchAll(\PDO::FETCH_ASSOC);
And for mysqli something like this (though I'm not too familiar with it)
$idUsers = $_SESSION['id'];
$ModuleID = 5;
$checkPoint = 999;
$query= "INSERT INTO userTakingModule (idUsers, ModuleID, checkPoint) VALUES (?, ?, ?) ON DUPLICATE KEY UPDATE idUsers = VALUES (idUsers), ModuleID = VALUES (ModuleID), checkPoint = VALUES (checkPoint)";
$stmt = $conn->prepare($query);
$stmt->bind_param("iii", $idUsers, $ModuleID, $checkPoint);
$stmt->execute();
$result = $stmt->get_result();