Logic like Formula

I have 3 Table. Order, Order Item and Item
Order and Item are Parents, Order Item is the child.

logic : if Order ID’s are similar give me total amount of this order.

Which formula to use and how to do it?

C’mon man, focus on the first one and be patient

[Total Amount Problem](https://community.appsheet.com/t/total-amount-problem/54477/14) Questions

Assuming that “Preliminary invoice” is a column from Order table, please share a screenshot of the Order table in order to see were the ID of that table is. Thanks