subtract amount of two fields

subtract amount of two fields

[tblPurchaseOrder.Amount]*.12 as VAT, [tblPurchaseorder.Amount] - [Vat] as exVat, tblPurchaseorder.Amount

but exVat is = tlbpurchase.Amount it doesnt subtract tblpurchaseorder.Amount*0.12

whole sql querry

[tblPurchaseOrder.Amount]*.12 as VAT, [tblPurchaseorder.Amount] - [Vat] as exVat, tblPurchaseorder.Amount

SELECT tblPurchaseOrder.ProductID, tblPurchaseOrder.Description, tblPurchaseOrder.Quantity, tblPurchaseOrder.Unit, tblPurchaseOrder.UnitPrice, tblPurchaseOrder.PreparedBy, tblPurchaseOrder.ApprovedBy, tblPurchaseOrder.PurchaseOrderNo, tblPurchaseOrder.Status, [tblPurchaseOrder.Amount]*.12 as VAT, [tblPurchaseorder.Amount] - [Vat] as exVat, tblPurchaseorder.Amount
FROM tblPurchaseOrder INNER JOIN tblSupplier ON tblPurchaseOrder.SupplierID = tblSupplier.SupplierID
WHERE (((tblPurchaseOrder.Status)="Approved")) and  (((tblPurchaseOrder.quantity)<>0))   ;