How to determine salaries greater than the average salary

Following shall work for you.

SELECT salary FROM table_name WHERE salary > (SELECT AVG(salary) FROM table_name);

select e.employee_id, e.department_id, e.salary from employees e
where salary > (
select  avg(salary)
from employees d where e.department_id =d.department_id)

SELECT * FROM table_name WHERE salary > (SELECT AVG(SALARY) from table_name);