order by yerine where mi kullanman lazımdı??

$vericek = $db->query("SELECT * FROM siparis_detay ORDER BY siparis_id='".$siparis_id."' desc limit 15", PDO::FETCH_ASSOC);

$vericek = $db->query("SELECT * FROM siparis_detay where siparis_id='".$siparis_id."' Order By urun_id desc limit 15", PDO::FETCH_ASSOC);

şeklinde günceller misin?