Why can't I run two mysqli queries? The second one fails [duplicate]

Solution 1:

It is possible with mysqli_multi_query().

Example:

<?php

$mysqli = new mysqli($host, $user, $password, $database);

// create string of queries separated by ;
$query  = "INSERT INTO images (project_id, user_id, image_name, date_created, link_to_file, link_to_thumbnail, given_name) VALUES ('$project_id', '$user_id', '$image_name', '$date_created', '$link_to_file', '$thumbnail', '$ImageName');";
$query .= "INSERT INTO images_history (project_id, user_id, image_name, date_created, link_to_file, link_to_thumbnail, given_name, day, month, year) VALUES ('$project_id', '$user_id', '$image_name', '$date_created', '$link_to_file', '$thumbnail', '$ImageName', '$day', '$month', '$year');";

// execute query - $result is false if the first query failed
$result = mysqli_multi_query($mysqli, $query);

if ($result) {
    do {
        // grab the result of the next query
        if (($result = mysqli_store_result($mysqli)) === false && mysqli_error($mysqli) != '') {
            echo "Query failed: " . mysqli_error($mysqli);
        }
    } while (mysqli_more_results($mysqli) && mysqli_next_result($mysqli)); // while there are more results
} else {
    echo "First query failed..." . mysqli_error($mysqli);
}

The key is that you must use mysqli_multi_query if you want to execute more than one query in a single call. For security reasons, mysqli_query will not execute multiple queries to prevent SQL injections.

Also keep in mind the behavior of mysqli_store_result. It returns FALSE if the query has no result set (which INSERT queries do not) so you must also check mysqli_error to see that it returns an empty string meaning the INSERT was successful.

See:
mysqli_multi_query
mysqli_more_results
mysqli_next_result
mysqli_store_result

Solution 2:

Some answers on Stack Overflow are so self-contradicting that it's just mind-blowing.

The key is that you must use mysqli_multi_query if you want to execute more than one query in a single call. For security reasons, mysqli_query will not execute multiple queries to prevent SQL injections.

It basically says, "The key is that you must use a firearm without a safety catch, because a regular weapon won't let you to shoot yourself in a foot. So here is the way to break it down and now you can cripple yourself in a single shot!"

Despite the fact the OP didn't ask how to run two queries in a single call, despite citing the explicit warning that the ability to run multiple queries in a single call is inherently dangerous, the answer nonchalantly provides the way to circumvent this limitation.

The worst part, all this dangerous and toilsome mess is for naught. Simply because there is not a single reason to run several queries in a single call. Running queries one by one is how a database API is meant to be used.

Basically I want to update two tables in my DB. Is there a better way to do this?

Yes of course. Just use two prepared queries.

$stmt = $dblink->prepare("INSERT INTO images 
(project_id, user_id, image_name, date_created, link_to_file, link_to_thumbnail, given_name) 
VALUES (?,?,?,?,?,?,?)");
$stmt->bind_param("ssssss", $project_id, $user_id, $image_name, $date_created, $link_to_file, $thumbnail, $ImageName);
$stmt->execute();

$stmt = $dblink->prepare("INSERT INTO images_history 
(project_id, user_id, image_name, date_created, link_to_file, link_to_thumbnail, given_name, day, month, year)
VALUES (?,?,?,?,?,?,?,?,?,?)");
$stmt->bind_param("ssssssssss", $project_id, $user_id, $image_name, $date_created, $link_to_file, $thumbnail, $ImageName, $day, $month, $year);
$stmt->execute();

it is not only much cleaner but also 100% safe from SQL injection.

And if one of your queries fails, simply ask mysqli for the error message and then fix the error.