Hi Frank,
You can see realtions between tables diagram below
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COM_Order -> COM_OrderItem (OrderItemOrderID = OrderID)
COM_OrderItem -> COM_SKU (OrderItemSKUID = SKUID)
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I hope it helps,
You can generate custom report with SQL,
but there are multiple reporting in "Store Reports"
It might be the one "Top Products by number of purchases" what you are looking for.
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