Opening Stock + Deliveries - Invoice = Closing Stock

Hi,
Please i need solution to the following:
I have the following Tables in My Database :
OpeningStock : (ID, ProductID, Qty, DateAdded) ProductID is foriegn to Products Table.
Delivery Details(DeliveryID, ProductID,Qty,Price) DeliveyID is foriegn to Delivery Table, ProductID is foriegn to Products Table.
Invoice Details(InvoiceID, ProductID,Qty,Price)InvoceID is foriegn to Invoice Table, ProductID is foriegn to Products Table.
Products(ID, Name, Price, CategoryID)

I will explain what i want to achive with a suedo code.
Opening Stock + Deliveris - Invoice = Closing Stock
Where Closing Stock is Stock Balance in store as close of business.

Please how do i writre the sql statement to get this done ?
...