sum for each row in related table based on current row

[florjan]

Hi everyone! Don't even know how to name this thread. 
Anyway, I have table workorders and for each workorder there is one or multiple documents from documents table and for each document there are entries from entries table where I choose material and quantity; positive or negative.

What I want to achieve is to get a table for specific workorder in which I have all the materials from all documents for this specific workorder with calculated final quantities.

I hope you can understand what I want to achieve.

For now I got a child table in workorder where all materials for that workorder are listed:

SELECT(entries[material], IN([document],SELECT(documents[ID], AND([workorder]=[_THISROW].[ID],OR([type]="outbound",[type]="inbound")))))

Now the hard part are quantites, and I have no idea how to attach final quantity for each material in that child child table. I tried with virtual column in materials table but I don't know how to formulate a formula. It should pull values based on parent parent table.

Can anyone help with that?