Show me document ids of documents with status working.
select document_id from documents where document_status_code = "working";
How about documents with the status done and type Paper?
select document_id from documents where document_status_code = "done" and document_type_code = "Paper";
Which of those were not shipped by agent named USPS?
select document_id from documents where document_status_code = "done" and document_type_code = "Paper" except select document_id from documents join ref_shipping_agents on documents.shipping_agent_code = ref_shipping_agents.shipping_agent_code where ref_shipping_agents.shipping_agent_name = "USPS";