Mysql Правильный запрос на обновление

opt={
"X6121Y-SU":"10.45",
"X6110CHIP-YLA":"3.80",
"X5400MDR-SU":"6.94",
"X3600PCR-SU":"6.46",
"X3420CHIP-HY":"3.70",
"X2125MDR-OS":"9.96",
"WXPCR":"9.32",
"WXBLADE":"2.36"
}

if(isset($_REQUEST['opt'])){
$data=json_decode($_REQUEST['opt']);
foreach ($data as $key=>$value) {
$db = Database::getInstance();
    $sth = $db->prepare("UPDATE " . config::$db_prefix2 . "product SET wholesale_price  = $value WHERE reference = '$key'");
$sth->execute();
}

}

$db = Database::getInstance();