Assume you have a table of a bill of materials (BOM):
BOM(ItemId, ParentItemId, Name, QtyPerParent)
Let's say that you need to get a count of child items to produce some root item. For this reason you need to multiply all QtyPerParent values in the production chain. Note that all ItemId's are unique. Here is the select:
select exp(sum(ln(QtyPerParent))) from ( select ItemId, QtyPerParent from BOM starting with ItemId = p_ChldItem connect by prior ParentItemId = ItemId minus select ItemId, QtyPerParent from BOM starting with ItemId = p_RootItem connect by prior ParentItemId = ItemId)
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This was first published in July 2002