Recursive Query

Recursive CTE is powerfull tool for bill of material calculations.

The query below calculates total parts, needed for top-level assemblies

declare  @BOM table(
id int
,parentId int
,qty float
)
insert @BOM values
( 20,10,2)
,(25,10,3)
,(30,20,5)
,(30,25,7)
,(40,30,11)

;with cte as(
	select [parentId] as assmblId, id, parentId, qty, qty as totalQty
	from @BOM 
	where [parentId] not in (select id from @BOM)
	union all
	select cte.assmblId, bom.id, bom.parentId, bom.qty, cte.totalQty * bom.qty
	from cte
	join @BOM bom on cte.id = bom.parentId
)
select assmblId, id, sum(totalQty) 
from cte
group by assmblId, id

Serg