Compute ordertotal from orderlines

Question

I'm new at this so sorry for the simple question.

I have 2 tables: orders and orderlines

orders.ordernumber

orders.ordertotal

 

orderlines.ordernumber

orderlines.orderline

orderlines.amount

 

I want to use the Computed Column Specification Formula to define the ordertotal from the orderlines. I tried something like:

SELECT SUM(amoumt) FROM Orderregels WHERE ordernumber=ordernumber

but that one returns an error.

 

How can I achieve what I want (if possible)?

 

Thnx in advance

 

Florian