Purchase order workflow query

Hi All,
I am using workflow for the purchase order approval. There are multiple level of approvers.
The customer requirement is to design a purchase order list page that will display the purchase order based on the users involved. Example: Submitter, Reviewer, Approver.
I wanted to do this using AOT query. However I am not able to apply union join between workflowTrackingStatusTable and WorkflowTrackingTable.
Please suggest any solution.
Thanks & Regards