我需要总结总计列的结果：

SELECT SUM(number_guest) AS total, booking_time
   FROM mt_bookingtable
   WHERE
            merchant_id IN (SELECT merchant_id 
                              FROM mt_merchant 
                              WHERE merchant_id = 1 )
            AND booking_time BETWEEN "12:00" AND "20:00"
            AND
            date_booking like "2020-06-01"
            AND status in ('pending','approved')
   GROUP BY booking_time

结果
我试着这样使用foreach：

foreach ($res as $item) {
    $sum += $item['total'];
    return $sum;
}

但我得到了结果 null .