What are the shipping agents' names?
select ref_shipping_agents.shipping_agent_name from ref_shipping_agents;
Show all the documents with shipping agent named USPS?
select * from documents as t1 join ref_shipping_agents as t2 on t1.shipping_agent_code = t2.shipping_agent_code where t2.shipping_agent_name = "USPS";
What is the name of the shipping agent of the document with id 2?
select ref_shipping_agents.shipping_agent_name from ref_shipping_agents join documents on documents.shipping_agent_code = ref_shipping_agents.shipping_agent_code where documents.document_id = 2;