CREATE TABLE Documents (Id VARCHAR2)
CREATE TABLE Ref_Shipping_Agents (Id VARCHAR2)

How many documents were shipped by 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"
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"