Oracle SQL

I have the following query that returns an error and i want to know what is the problem ? SELECT last_name, first_name FROM employees WHERE salary = (SELECT salary FROM employees WHERE department_id = 30); the error returned is : (SELECT salary FROM employees WHERE department_id ...

I am working on Oracle database 11g, and facing an issue while trying to execute the following query: SELECT d.department_name, e.manager_id, l.location_id, sum(e.salary) FROM departments d, employees e, locations l WHERE d.department_id = e.department_id AND e.manager_id = d.manager_id (+) AND l.location_id = d.location_id (+) GROUP BY ...