>>Yes, sure. JOIN with 2 derived tables or better use CTE to simplify the code.
>
>I don't know how CTE works (or what it is). But I will think on how to apply JOIN to this case. Thank you.

I updated my reply with the code, but didn't have a chance to post - BTW, I didn't get an error either.

Anyway, here is the code:

;with cteSum1 as (select 
PurchaseOrders.ProjectAccount, sum(PurchaseOrders.PoAmount) as TotPro 
from PurchaseOrders group by ProjectAccount)
 
update Projects set SpentTest = PoSum.TotPo 
from Projects  inner join cteSum1 PoSum
ON Projects.ProjectAccount = PoSum.ProjectAccount