Mysqli query is returning a null value for a simple row of strings from a DB

Question

I am trying to retrieve a single row from a MySQL table using a mysqli statement. I've tried several different iterations of code, subtly changing the structure based on various previous questions from this forum, and others, but can't seem to get any result other than 'null'.

This is part of a larger script which is called via an Ajax request with jQuery. I've included both the PHP and the Javascript below, though I'm fairly confident in the JS being OK (preparing to be told otherwise now...).

Any suggestions as to where I'm going wrong would be very much appreciated as I can't see the wood from the trees anymore, and am just going around in circles.

PHP:

//initiate new mysqli object
$retrieve_link = new AuctionMySQLi($db_host, $db_user, $db_password, $db_name); //custom subclass, this definitely works as is used in other scripts on the server

//prepares DB query. Query has been tested on phpmyadmin and returns the expected data set
$stmt = $retrieve_link->prepare("SELECT `item_number`,`item_name`,`item_category`,`end_date`,`auction_type`,`high_bid_number` FROM `item` WHERE `item_number`=2");

$stmt->execute(); //no params to bind, so execute straight away
$stmt->bind_result($item);
$stmt->fetch();
$dataset = $item->fetch_row();

$response[0] = $dataset; //returned data forms part of larger dataset
echo json_encode($response); //return the entire dataset to a jquery Ajax request
die;

JS:

//this definitely works as objects have been returned via the 'success' function as the code was being developed
$.ajax({
url         : "items/populate-home-page-script.php",
type        : "GET",
data        : {data:toSend},
dataType    : "json",
success     : function(data){
    alert(data[0]);
},
error       : function(jqXHR, textStatus, errorThrown){
    alert(textStatus+','+errorThrown);
}
});

return false;

I have also tried using fetch_assoc() and fetch_row() as part of the PHP query, taking direction from the PHP reference material here and here. I have also read through these questions from Stackoverflow this, this, and this, but I still seem to get a return of null for every different code combination I try.

As I've said in a code comment, I know that the link to the DB works as I've used it in other scripts, and in other areas in this script - so there's no reason why this object wouldn't work either. I also know that the query returns the expected data when inputted to phpmyadmin.

The returned data is just a number of strings, any all I would like to do is store around 16 returned datasets to an array, as part of a loop, and then return this array to the Ajax request.

Have you checked that there are no errors reported by MySQL after the query? — Tero Kilkanen, Feb 28 '14 at 02:21
@TeroKilkanen - no, and an excellent point that I had completely overlooked, I'll add in an error report at the execute() stage — dsaa, Feb 28 '14 at 02:28

Tomasz Rakowski · Accepted Answer · 2014-02-28T02:59:10.470

You are using "AuctionMySQLi" which appears to extend the regular Mysqli driver. I'll assume it does this correctly.

You're using prepared statements which is probably an overkill in this case. You could accomplish the same thing with something like this (php 5.3, mysqli + mysqlnd):

$retrieve_link = new AuctionMySQLi($db_host, $db_user, $db_password, $db_name);
$result = $retrieve_link->query("SELECT `item_number`,`item_name`,`item_category`,`end_date`,`auction_type`,`high_bid_number` FROM `item` WHERE `item_number`=2");
if($result !== false) {
    echo json_encode($result->fetch_all());
} else {
    echo json_encode(array());
}
$retrieve_link->close();

If you're using an older php version, or mysqlnd is not available, you can also do

$retrieve_link = new AuctionMySQLi($db_host, $db_user, $db_password, $db_name);
$result = $retrieve_link->query("SELECT `item_number`,`item_name`,`item_category`,`end_date`,`auction_type`,`high_bid_number` FROM `item` WHERE `item_number`=2");
if($result !== false) {
   $output = array();
    while($row = $result->fetch_assoc()) {
        $output[] = $row;
    }
    echo json_encode($output);
} else {
    echo json_encode(array());
}
$retrieve_link->close();

I also understand that you want to limit the number of results. In both cases, a good way of getting it done is to use a LIMIT statement in SQL. This is lower the overhead overall at source. Otherwise you can array_slice to slice the output of result->fetch_all() in solution 1, or $output in solution 2.

Finally, if you insist in using prepared statement read the note at http://ca2.php.net/manual/en/mysqli-stmt.bind-result.php and analyze provided example:

$retrieve_link = new AuctionMySQLi($db_host, $db_user, $db_password, $db_name);
$stmt = $retrieve_link->prepare("SELECT `item_number`,`item_name`,`item_category`,`end_date`,`auction_type`,`high_bid_number` FROM `item` WHERE `item_number`=2");
$stmt->execute();
$stmt->bind_result($itemName, $itemCat, $endDate, $auctionType, $highBidder);
$output = array();
while($stmt->fetch()) {
    $output[] = array($itemName, $itemCat, $endDate, $auctionType, $highBidder);
}
echo json_encode($output);
$retrieve_link->close()

accepted because the third code snipped produced results without having to change too much of my approach. That said, I take on board the over-kill of using prepared statements for this simple query. Unfortunately the resultant array is still showing null values, and I'm not sure why, however, the problem presented in the question has been solved successfully. — dsaa, Feb 28 '14 at 15:02

score 0 · Answer 2 · answered Feb 28 '14 at 02:37

It looks to me like you may have some confusion about ->fetch() and ->fetch_row(). You should use one or the other, but not both.

Try this to retrieve your result set:

$stmt->execute();
while ($dataset = $stmt->fetch_row()) {
    $response[] = $dataset; //returned data forms part of larger dataset
}

This will append each row of your result set to your $response array.

Mysqli query is returning a null value for a simple row of strings from a DB

2 Answers2