How many documents are there?
select count(*) from documents;
How many documents were shipped by Fedex?
select count(*) from ref_shipping_agents join documents on documents.shipping_agent_code = ref_shipping_agents.shipping_agent_code where ref_shipping_agents.shipping_agent_name = "Fedex";
How about USPS?
select count(*) from ref_shipping_agents join documents on documents.shipping_agent_code = ref_shipping_agents.shipping_agent_code where ref_shipping_agents.shipping_agent_name = "USPS";