Oracle PLSQL: ORA-00923 Error Message

ora 923ORA-00923: FROM keyword not found where expected Below is the SQL : SELECT A.Agent_key, Sum(Total_Orders) FROM (SELECT A.Agent_key, Sum(Total_Orders)What causes an ORA-00923 error? In a SELECT or REVOKE statement, the keyword FROM was either missing, misplaced, or misspelled. The keyword FROM must follow