SQL Command not properly ended?

oracle does not support as for table aliases, only for column aliases and they are optional for that use => delete all as keywords ;)

You shouldn't put the AS temp. When putting alias to a table (or subquery) you should only write the alias. This should work:

SELECT Temp.name, Temp.AvgSalary 
FROM ( SELECT A.aid, A.aname AS name, AVG(E.salary) AS AvgSalary 
       FROM Aircraft A, Certified C, Employees E)  Temp;

Best regards,