declare @qty int
set @qty=60
update test set tptalqty=(case when TotalQty - UnitQty * Qty>0
then TotalQty - UnitQty * Qty else 0 end)
set @qty=60
update test set tptalqty=(case when TotalQty - UnitQty * Qty>0
then TotalQty - UnitQty * Qty else 0 end)
then TotalQty - UnitQty * 60 else 0 end)
case
when TotalQty - UnitQty * Qty < 0 then 0
else TotalQty - UnitQty * Qty
end,