foto random con PDO

[LedGiallo]

Salve, ho questo codice che mi manca solo la parte della WHERE, che non so cosa metterci per far che mi carica ad random le foto che si trovano nelle sezioni diverse.

come faccio?
ecco il codice:

<?php 
$ok = false;
$stmt = $db->prepare("SELECT photo_categories.title as _title,  photo_categories.id as _id,  photo_categories.dir as _dir , photo.*, DATE_FORMAT(photo.data_reg,'%d/%M/%Y ore %H:%i') as 'data_it' FROM photo
           INNER JOIN photo_categories ON photo_categories.id = photo.id_categories WHERE  photo.active=1 and "); 
$stmt->execute(); 
?>
<div class="menu">
<ul>
<?php
while(($row = $stmt->fetch()) !== false) {
  $ok = true;
?>
<li><a href="photo_view.php?id=<?php echo "".$row["id"].""; ?>"><img src="<?php echo $dir_photo; ?><?php echo $row["_dir"];?>/thumb/<?php echo $row["url"]; ?>"></a></a></li>
<?php
}             
?>
</ul>
<?php   
if (!$ok) {
echo '404';
}
?>
</div>

avete idea come fare?

grazie mille e buona serata.

[LedGiallo]

ok, trovato e risolto.
$stmt = $db->prepare("SELECT photo_categories.title as _title, photo_categories.id as _id, photo_categories.dir as _dir , photo.*, DATE_FORMAT(photo.data_reg,'%d/%M/%Y ore %H:%i') as 'data_it' FROM photo
INNER JOIN photo_categories ON photo_categories.id = photo.id_categories WHERE photo.active=1 ORDER BY RAND() LIMIT 1");