使用计数器变量获取列中的下一个值，而SQL中没有任何数组？

I want to move to next row in a column using a variable. Like I have a column dr. Now when I have done calculations to first value of dr , I want to go to next value in the column, and perform some calculations there, and then to third and so on.

Algorithm

1. Get the value of $paymentfrom the user.
2. Select $dr from a table balance.
3. Calculate $debited_amount = $dr- $payment. There can be three cases.

   a) $debited_amount< 0 // when the user pays more amount as he needs to pay for gp_no = 1

   b) $debited_amount > 0) // when the user pays less amount as he needs to pay for gp_no = 1

   c) $debited_amount = 0 // when the user pays exactly the same amount as he needs to pay for gp_no = 1

4. If (c) , then update balance by setting $dr= 0 for first group and exit;
5. If (b) , then update balance by setting $dr= $debited_amount for first group and exit;
6. If (a) , then update balance by setting $dr= 0 for first group and select dr for next group and goto step3 ;

Below is the PHP code:

<?php

 If(isset($_REQUEST['submit'])!='')
{

$member_no = $_REQUEST['member_no'];
$payment = $_REQUEST['payment'];



$sql = "create view balance as select gp_no,sum(dr) as dr from instalment WHERE  member_no = '$member_no' group by gp_no" ;
$sql = "select * from balance";
$result = mysqli_query($conn, $sql);

if (mysqli_num_rows($result) > 0)
{
while($row = mysqli_fetch_assoc($result))
    {

     $dr = $row["dr"];

    }
}

$debited_amount = $dr - $payment ; 
$payment = $payment - $dr;


$i = 1 ;
do 
 {

     if ($debited_amount < 0 )   // when the user pays more amount as he needs to pay for gp_no = 1
     {
          $sql = "update balance set dr = 0 WHERE  gp_no = '$i' " ;// dr will be updated by dr = 0;
           $dr++; // and move to next dr for subratraction i.e., dr of gp_no = 2 
          $i++;
          $debited_amount = $dr - $payment ;        
          $payment = $payment - $debited_amount ;       
     }
     elseif ($debited_amount > 0)  // when the user pays less amount as he needs to pay for gp_no = 1
     {
         $sql = "update balance set dr = $debited_amount WHERE  gp_no = '$i' " ;// dr will be updated by dr = $debited_amount ; 
         $dr = $debited_amount ; 
         $payment = $payment + $debited_amount ;
     }
     else  // when the user pays exactly the same amount as he needs to pay for gp_no = 1
     {
         $sql = "update balance set dr = 0 WHERE  gp_no = '$i' " ;// dr will be updated by dr = 0;
         $dr = 0;
     }


} while ($payment !=0 );