Aggregate function results in select statement - sql

Question

Hopefully the code below should demonstrate what I'm trying to achieve.
The issue is that none of the input selects are resolved by the time I try to calculate VatableCash so I get "Invalid Column" when trying to select it.
Sorry if there's something plainly obvious I can do here. SQL isn't one of my strong suits.
select
OrderHeader.ID,
sum(OrderLine.NetPrice) as OrderLineNetPrice,
sum(OrderLine.GrossPrice) as OrderLineGrossPrice,
sum(
case when PaymentOption_ID = 8
then Payment.Amount
else 0
end
) as TotalCashAmount,
((OrderLineGrossPrice - OrderLineNetPrice) / OrderLineGrossPrice) * TotalCashAmount as VatableCash
from OrderHeader
inner join Payment on Payment.OrderHeader_ID = OrderHeader.ID
inner join OrderLine on OrderLine.OrderHeader_ID = OrderHeader.ID
group by OrderHeader.ID

Answer 1

You need to use sub query.
you can try this.
;WITH CTE AS
(
select
OrderHeader.ID,
sum(OrderLine.NetPrice) as OrderLineNetPrice,
sum(OrderLine.GrossPrice) as OrderLineGrossPrice,
sum(
case when PaymentOption_ID = 8
then Payment.Amount
else 0
end
) as TotalCashAmount
from OrderHeader
inner join Payment on Payment.OrderHeader_ID = OrderHeader.ID
inner join OrderLine on OrderLine.OrderHeader_ID = OrderHeader.ID
group by OrderHeader.ID
)
SELECT *,
((OrderLineGrossPrice - OrderLineNetPrice) / OrderLineGrossPrice) * TotalCashAmount as VatableCash
FROM CTE

Answer 2

Love the cross apply! Use it whenever you want some handy extra columns.
select
OrderHeader.ID,
sum(OrderLine.NetPrice) as OrderLineNetPrice,
sum(OrderLine.GrossPrice) as OrderLineGrossPrice,
TotalCashAmount,
((OrderLineGrossPrice - OrderLineNetPrice) / OrderLineGrossPrice) * TotalCashAmount as VatableCash
from OrderHeader
inner join Payment on Payment.OrderHeader_ID = OrderHeader.ID
inner join OrderLine on OrderLine.OrderHeader_ID = OrderHeader.ID
cross apply ( select sum(
case when PaymentOption_ID = 8
then Payment.Amount
else 0
end
)) as subquery(TotalCashAmount)
group by OrderHeader.ID