Filemaker simple function

I am looking for a Filemaker developer.
I do not want to assign a larger project without some personal history with the person i will be working with.
This request is not for the whole project, just a for a function and if you really know Filemaker code this should be really easy task.

The initial task and your bid applies to:
I have 2 tables "Orders" and "Inventory".
I want add a column "Sold Past 30 Days" to "Inventory" that get the sales history from the past 30 days. All products in "Inventory" have a unique ID that can match from "Orders". "Orders" table include all neccecery data like "OrderDate", "QtySold". I want the sum of the "QtySold" past 30 days.

Matching ID:
Lager::Property EAN
Order::identifier

Needed:
Order::created at
Order::quantity

Result should be showed in: 

Lager::SoldQty30Days 


Something like SQL-ish(i am no coder):
Lager::SoldQty30Days =
SELECT SUM (Order::quantity)
FROM Order
JOIN Lager
ON Order::identifier = Lager::Property EAN
WHERE Lager::Property EAN = Order::identifier AND Order::created <= (30daysMax)

Filemaker project containing data will be provided.