I have following view in mariadb which gives stock credit, debit and balance :

SELECT 
       pitem,
       Bdate,
       credit,
       debit, 
       sum(ifnull(bal, 0)) OVER (PARTITION BY pitem ORDER BY bdate, DESC1) balance, 
   DESC1
FROM 
(
SELECT a.Pitem,
       a.Bdate,
       a.Trn,
       If((trn=1
           AND a.qty>0), a.Qty, 0) AS Credit,
       If(trn=0
          OR a.qty<0, abs(a.Qty), 0) AS Debit,
       (If((trn=1 AND a.qty>0), a.Qty, 0)) - (If(trn=0 OR a.qty<0, abs(a.Qty), 0)) AS Bal,
       a.Desc1
FROM
  (SELECT tblphmrefill.RfItem AS PItem,
          tblphmrefill.rfDate AS BDate,
          tblphmrefill.rfQty AS Qty,
          CONCAT(if(rfqty>0, 'Purchase No_', 'Discard_BMW_'), coalesce(tblphmrefill.RfInvoice, 0), '_Vend_', rfsuppID) AS DESC1,
          1 AS Trn
   FROM tblphmrefill
   UNION SELECT invoicerefundphm.phmItem AS pItem,
                invoice.Bdate,
                invoicerefundphm.Qty,
                CONCAT('Refund_', billno, '_', invoice.BName) AS DESC1,
                1 AS Trn
   FROM invoice
   INNER JOIN invoicerefundphm ON invoice.BilID = invoicerefundphm.Billno
   UNION
     (SELECT invoicephm.phmItem AS PItem,
             invoice.BDate,
             invoicephm.Qty,
             CONCAT('Sales_', billno, '_', invoice.BName),
             0 AS Trn
      FROM invoicephm
      INNER JOIN invoice ON invoicephm.Billno = invoice.BilID ORDER BY invoice.bdate, invoice.bilid ASC)) AS a )
      phmtbatch_int 
Order by Bdate, DESC1 Asc 

The problem is this part :

SELECT a.Pitem,
     a.Bdate,
     a.Trn,
     If((trn=1
         AND a.qty>0), a.Qty, 0) AS Credit,
     If(trn=0
        OR a.qty<0, abs(a.Qty), 0) AS Debit,
     (If((trn=1 AND a.qty>0), a.Qty, 0)) - (If(trn=0 OR a.qty<0, abs(a.Qty), 0)) AS Bal,
     a.Desc1

here the code is calculating balance but running the same calculation twice.

I have tried following thinng:

SELECT a.Pitem,
       a.Bdate,
       a.Trn,
       @Credit := (If((trn=1 AND a.qty>0), a.Qty, 0)) AS Credit,
       @Debit := (If(trn=0 OR a.qty<0, abs(a.Qty), 0)) AS Debit,
       (@Credit - @Debit) AS Bal,
       a.Desc1

but mariadb throws an error 1351 saying view contains variable or parameter which is not allowed.
is there any way to avoid calculating credit, debit twice for balance?