Mariano has a new post with a look at Year-to-year Inventory Margin Report using the PIVOT operator in T-SQL
He then updates it with follow up post that makes it work even better.
The Dynamics GP Blogster: Year-to-year Inventory Margin Report using the PIVOT operator in T-SQL
SQL & DataSales, Purchasing & InventoryReporting & BI
Archived post from 2011. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.