Question

I feel this is a more logic problem than anything. A database has pictures saved via a source reference and booleans for tags e.g. isLandscape=1. I had made a system to traverse pages of results based on types asked. The following is an example of what I’m facing. I only see the same 12 pictures from page 0 -> page 22. Then I start to see new ones. I think I have just been overlooking this bug since I had not noticed it until now. One thing I noticed was page22*12pictures = 264 which is the same as the first new picture id that is seen. You can see the error here (just change the p to different pages).

<?php
$pictureid   = -1;
$startpage   = 0;
$viewsection = -1;
$uid         = -1;  //user id

$amntperrow  = 4; //how many pictures per row, must correlate with doThumb()'s switch case amounts
$maxrows     = 3;    //how many rows of pictures to drop

if(isset($_GET['pid']) && is_int(intval($_GET['pid']))) $pictureid   = clean($_GET['pid']);
if(isset($_GET['sec']) && is_int(intval($_GET['sec']))) $viewsection = clean($_GET['sec']);
if(isset($_GET['p'])   && is_int(intval($_GET['p'])))   $startpage   = clean($_GET['p']);

$result = generateResult(array("isFlowers"), $startpage);
//**snip** -- drawing thumbnails would happen here

function generateResult($types, $page) {
    global $amntperrow;
    global $maxrows;

    $sqlWheres = "";
    $idAmnt = ($amntperrow*$maxrows)*$page;

    if(isset($types) && !empty($types)) {
        if(count($types) >= 1) {
            for($i = 0; $i<count($types); $i++) {
                $sqlWheres .= $types[$i] . "='1'";
                if($i < count($types)-1) $sqlWheres .= " AND ";
            }
        }
    }        

    $result = "SELECT * FROM pictures WHERE ";
    if(!empty($sqlWheres)) $result .= $sqlWheres . " AND " ;
    $result .= " private='0' AND id >='" . $idAmnt . "' LIMIT " . ($amntperrow*$maxrows);

    return $result;
}
?>

This seems like a glaring bug that I am overlooking. Thanks for the help.

Answer 1

What is the difference between these two queries?

SELECT *
FROM pictures
WHERE private = '0' AND id >= '24'
LIMIT 12;

and

SELECT *
FROM pictures
WHERE private = '0' AND id >= '36'
LIMIT 12;

Answer: potentially no difference at all. The database engine can decide in either case that it wants to return pictures with ids 100 through 111 – that result set meets all of the conditions of either query.

Try a query like this instead:

"SELECT *
FROM pictures
WHERE private = '0'
ORDER BY id
LIMIT " . $idAmnt . ", " . ($amntperrow * $maxrows)

The ORDER BY id is really the key. Paging through database results is generally done with a combination of ORDER BY and LIMIT.