Given Invoice#, find original Order#

Hi,

maybe a simple thing but I can't seem to find any way to do this.

Say I have an OrderId for CartType 'O' that was delivered and invoiced already.

I can find the Delivery document in Webtools by joining on the Shipment/ShipmentDetails table, but I can't find the correspomding Invoice - as it's also just an Order with apparently no link back to even its delivery.

i.e in B1 given a Sales Order, I can find the Delivery Note, from this I can find the Invoice. But there doesn't seem to be any such facilty via Webtools?

Is it possible without having to create and maintain my own lookup table?

thx,

Steve