我有這個問題，首先要獲取學生總數，然后是 amount_paid 列中學生支付的總金額，我正在查詢所有學生 id(system_id)，然后是每筆付款，然后是發票和發票的成本費用目標是與學生匹配的 id。最后，我嘗試減去循環中發送給選定學生的所有發票的總金額，然后從同一循環中發送給目標學生的費用中減去它?，F在，我的許多問題是，對于第一批學生來說，它正在計算余額罰款，但其余的余額正在迅速增加。請問我的代碼問題出在哪里：<?php$students = mysqli_query($conn, "SELECT * FROM students GROUP BY system_id");while($row = mysqli_fetch_assoc($students)){    $user_uid = $row['system_id'];    $exam = $row['exam_number'];   //getting payments balances//gettingshopping cart details      $Balance_query = mysqli_query($conn,  "SELECT SUM(amount_paid) AS 'sumitem_cost' FROM payments WHERE payment_by='$user_uid' ");          $balance_data = mysqli_fetch_array($Balance_query);      $balance_price = $balance_data['sumitem_cost'];            $py = mysqli_query($conn, "SELECT * FROM payments WHERE payment_by='$user_uid' AND status!='rejected' GROUP BY invoice_id");      while($rowpy = mysqli_fetch_assoc($py)){                    $paidAmout = $rowpy['amount'];          $invoiceId = mysqli_real_escape_string($conn, $rowpy['invoice_id']);          $PaymentStatus = mysqli_real_escape_string($conn, $rowpy['status']);          //Getting invoice          $Invoice = mysqli_query($conn, "SELECT * FROM invoices WHERE id='$invoiceId'");                    while($rowInv = mysqli_fetch_assoc($Invoice)){          $NewFeeId = $rowInv['id'];                    $sql = mysqli_query($conn,"SELECT SUM(fee) as total FROM invoices WHERE id='$invoiceId'");         $row = mysqli_fetch_array($sql);          $sum = $row['total'];          $total_price += $row[‘fee’];      }}     echo'<br>'.$exam.':' . $BalanceToPay =  $total_price - $balance_price;      }?>