Hi,
I have a parent-child relationship table, which already merge into one signle cursor. I need to deduct it's qty accordingly. How should I SQL it?

Example
My cursor
Parent ID    ChildID       UsedQty
A01          Z01               7      
Z01          X01               5
X01          Y01               8

A01 contains 7 pcs of item Z01. 
Z01 contains 5 pcs of item X01.
X01 contains 8 pcs of item Y01.

I need to calculate the current qty of each item which,

Current Item Qty Before Deducted
--------------------------------
ItemID      Qty
A01         10
Z01          2
X01         12
Y01          6

Expect Result
ItemID       Qty
A01          10
Z01           2 - (10* 7)
X01          12 - ( 2* 5)
Y01           6 - (12* 8)

How to qrite SQL to make it?

Thank you