Hi Naman,
Datamart NBI_RT_FACTS would not show any changes in the Department historically as this is not an audit table. It would only show the current department for this resource.
Instead of querying Datamart, consider using Audit trail on the Resource object and setting up Attribute audit on Department OBS, example for Update Audit:
This will make historical data on any changes on the Department OBS available in:
PPM UI on Home - Resources - Resource - Audit tab
Audit Trail portlet in Administration
CMN_AUDITS table.
To enable it go to Administration - Objects - Resource - Audit Trail, move the attributes over as above and save, then test.
Hope this helps -Nika