How to Improve This Self-Joins - sql

Question

I am learning Oracle SQL by working with its primitive HR schema where there is EMPLOYEES table which has three columns that I'm mainly interested in: MANAGER_ID, which is basically a self reference to EMPLOYEES.EMPLOYEE_ID, DEPARTMENT_ID, and SALARY. (You can find the schema diagram and schema objects here).
I wish, for each employee, to retrieve his/her SALARY, alongside of employee's manager's departmental average salary. For instance, if we have the following (EMPLOYEE_ID = 140 is the interested party here):
+-------------+--------+---------------+------------+
| EMPLOYEE_ID | SALARY | DEPARTMENT_ID | MANAGER_ID |
+-------------+--------+---------------+------------+
| 140 | 12000 | 50 | 110 |
| 110 | 20000 | 60 | 101 |
| 156 | 18000 | 60 | 101 |
| 175 | 15000 | 60 | 105 |
| 320 | 24000 | 60 | 105 |
+-------------+--------+---------------+------------+
I am interested in obtaining an average salary of all the managers (not all other non-managerial employees) in department where employee's manager works at (in this case, DEPARTMENT_ID =60), and compare it with employee's (in this case, 140). In a sample data above, the output should be:
+-------------+--------+-------------+-------------+------------+
| EMPLOYEE_ID | SALARY | AVG_MGR_SAL | MGR_DEPT_ID | MANAGER_ID |
+-------------+--------+-------------+-------------+------------+
| 140 | 12000 | 19250 | 60 | 110 |
+-------------+--------+-------------+-------------+------------+
where we have four (4) managers working in department 60, and $19250 being calculated as (20000 + 18000 + 15000 + 24000) / 4. I have come up with the following query that seems to work (and excludes those employees that don't have a manager):
select
employee_id
, salary employee_salary
, trunc(mgr_info.avg_manager_salary_per_dept, 0) emp_manager_avg_sal_dept
, mgr_info.manager_dept_id
, mgr_info.manager_id
from employees
join (
select
e1.employee_id manager_id
, e1.department_id manager_dept_id
, e1.salary manager_salary
, avg(e1.salary) over (partition by e1.department_id) avg_manager_salary_per_dept
from employees e1
join (
select distinct manager_id
from employees
where manager_id is not null
) mgr_ids
on e1.employee_id = mgr_ids.manager_id
) mgr_info
on employees.manager_id = mgr_info.manager_id
order by employee_id
However, I feel like that there should be a better way of getting the same result with fewer self-joins. Is there a way to get a better performance?

Answer 1

Something like this... You only need one join, you can compute the average salary for the manager's department on the "manager" copy of the table. I only included a few columns, you may need more, or fewer, but I believe the core of what you wanted is covered.
(NOTE: Edited since I realized I missed one detail in the requirement)
select e.employee_id as employee_id,
e.salary as employee_salary,
m.employee_id as manager_id,
m.department_id as manager_dept_id,
m.avg_salary as avg_sal_of_mgr_dept
from hr.employees e inner join
( select employee_id, department_id,
avg(salary) over (partition by department_id) as avg_salary
from hr.employees
where employee_id in (select manager_id from hr.employees)
) m
on e.manager_id = m.employee_id
;

Answer 2

Here is an option which uses a series of joins to get your result:
SELECT DISTINCT t1.EMPLOYEE_ID,
t1.SALARY,
t1.DEPARTMENT_ID,
COALESCE(t2.SALARY, 0.0) AS ManagerAvgSal
FROM employees t1
LEFT JOIN
(
SELECT e1.DEPARTMENT_ID, AVG(e1.SALARY) AS SALARY
FROM employees e1
WHERE e1.EMPLOYEE_ID IN (SELECT DISTINCT MANAGER_ID FROM employees)
GROUP BY e1.DEPARTMENT_ID
) t2
ON t1.DEPARTMENT_ID = t2.DEPARTMENT_ID