Table schema:
CREATE TABLE Documents (Id VARCHAR); CREATE TABLE Ref_Shipping_Agents (Id VARCHAR)

Question: How many documents were shipped by USPS?
SQL: 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"