没有子查询的“子查询返回多于1行”的MySQL？

Question

I have a webapp that when a link for a document is clicked, a PHP script is called that updates the "viewed" status for the document in the DB. This is the script that is called when the click happens:

<?php
include '../../variables.php';

// The document that is passed through POST
$document = $_POST['document'];

$conn = new mysqli($dbhost, $dbuser, $dbpassword, $db);

if (!$conn){
    die('Could not connect to db: '.mysqli_error($conn));
}

$sql = "UPDATE files
        SET docViewed = '1'
        WHERE fileloc = '$document'";
$query = mysqli_query($conn, $sql);

if (!$query){
    die('Could not update docViewed: '.mysqli_error($conn));
}
?>

As you can see, I have no subquery in the MySQL query that updates the field I want, yet I still am receiving this error:

Could not update docViewed: Subquery returns more than 1 row

I have tried appending to the query:

"...
WHERE fileloc = '$document'
LIMIT 1";

However, I still get the same result.

To be clear, each $document has to be UNIQUE in the database, so there are no duplicate entries.

UPDATE: This post is not a duplicate of the suggested post, as that OP is using a subquery. In this example, I am not using a subquery anywhere.

Here is the structure of the files table I use. Also to show there is no duplicate of $document , I filtered the table by the fileloc which is 30294/1506012960606.pdf :

UPDATE 2: I have narrowed down the actual MySQL query that is happening to produce this error:

UPDATE files
SET docViewed = '1'
WHERE fileloc = '30294/1492682311085.pdf'

Answer 1

This doesn't change too much, but adds some more logging points to maybe drill down on where the issue is. I updated your mysqli usage to the object oriented approach, as well as parameterizing the queries using prepared statement (always good to avoid sql injection, but in this case extra practical because it lets us test the query in several steps).

<?php
try {
    include '../../variables.php';

    // The document that is passed through POST
    $document = filter_input(INPUT_POST, 'document', FILTER_SANITIZE_STRING);

    $conn = new mysqli($dbhost, $dbuser, $dbpassword, $db);

    if ($conn->connect_error){
        throw new Exception("({$conn->errno}) {$conn->error}");
    }

    $sql = "UPDATE files
            SET docViewed = '1'
            WHERE fileloc = ?";
    $stmt = $conn->prepare($sql);

    if (!$stmt) {
        throw new Exception("({$conn->errno}) {$conn->error}");
    }
    $stmt->bind_param('s', $document);
    $exec = $stmt->execute();

    if (!$exec) {
        throw new Exception($stmt->error);
    } else if ($stmt->affected_rows === 0) {
        throw new Exception('No file location found');
    }


} catch (Exception $e) {
    error_log($e);
    die($e->getMessage());
}
?>