Calculating the total balances of the students in PHP and MYSQL - php

Question

I have this problem, first am getting the total students and then the total amount paid by students in column amount_paid and I am querying all the students id(system_id) and then the payments each made and then the invoices and the cost fee of the invoices target to the id’s matching the student.
And then at last am trying to subtract the total amount paid to all the invoices sent to a selected student in a loop and then subtract it from the fees sent to targeted students in that same loop. Now my many issue is for the first students it’s calculationg the balance fine but with the rest the balances are rapidly increasing. Please where is the issue with my code:
<?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;
}
?>

Answer 1

You have some weird queries. Single quotes should never be used for column aliases and why do you group by system_id?
Your calculations would be greatly simplified by using JOINs and if I were your teacher I'd expect to see joins.
This will solve the assignment at once:
SELECT s.system_id
, sum(amount_paid) as sumitem_cost
, sum(fee) as total
FROM students s
JOIN payments p on s.system_id = p.payment_by
JOIN invoices i on p.invoice_id = i.id

Answer 2

You need to zero (init) $total_price every time you deal with new student. You do not do that explicitely, so on first use of $total_price it's created zeroed, but then you iterate further and keep adding to it, ending up with cumulative value. So simply add
$total_price = 0;
for each while loop iteration:
while($row = mysqli_fetch_assoc($students)){
$total_price = 0;
...

Answer 3

you can try this code
<?php
$students = mysqli_query($conn, "SELECT * FROM students GROUP BY system_id");
$grandBalanceForAllStudent = 0 ;
while($row = mysqli_fetch_assoc($students)){
$user_uid = $row['system_id'];
$exam = $row['exam_number'];
$total_price = 0 ;
//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’];
}}
$BalanceToPay = $total_price - $balance_price;
// you can keep for your data
$grandBalanceForAllStudent += $BalanceToPay ;
echo'<br>'.$exam.':' . $BalanceToPay ;
}
?>