Aggregate function and join on 2 conditions - sql

Question

I have this task for Oracle. It requires me to display the departments names(department is a table containing the manager_id for that table, department_id and department_name), the manager_id for that department, the name of the manager(found in employees table) and the average salary for that department(salary of each employee is also found in employees). As soon as I try to retrieve the manager name(I suppose by comparing his id in dept with the one in employees) it messes up my averages. Without it(Like following) it works just fine
SELECT d.department_name, AVG(e.salary) as "Salaries"
FROM employees e join departments d on e.department_id=d.department_id
WHERE d.manager_id=e.employee_id
GROUP BY e.department_id, d.department_name,d.manager_id
ORDER BY AVG(e.salary)
Can someone help me solve this and perhaps explain why I mess it up?

Answer 1

WITH factoring clause selects average salary; it is then joined to other tables
SQL> with avgsal as
2 (select e.department_id, round(avg(e.salary)) avg_sal
3 from employees e
4 group by e.department_id
5 )
6 select d.department_id,
7 d.department_name,
8 m.last_name manager,
9 a.avg_sal
10 from departments d
11 join employees m on m.department_id = d.department_id
12 and m.employee_id = d.manager_id
13 join avgsal a on a.department_id = d.department_id
14 order by a.avg_sal;
DEPARTMENT_ID DEPARTMENT_NAME MANAGER AVG_SAL
------------- -------------------- ------------------------- ----------
50 Shipping Fripp 3476
30 Purchasing Raphaely 4150
10 Administration Whalen 4400
60 IT Hunold 5760
40 Human Resources Mavris 6500
100 Finance Greenberg 8601
80 Sales Russell 8956
20 Marketing Hartstein 9500
70 Public Relations Baer 10000
110 Accounting Higgins 10154
90 Executive King 19333
11 rows selected.
SQL>
A quick check for 2 departments:
SQL> select department_id, round(avg(salary)) avg_sal, count(*) "# of employees"
2 from employees
3 where department_id in (20, 30)
4 group by department_id
5 order by 1;
DEPARTMENT_ID AVG_SAL # of employees
------------- ---------- --------------
20 9500 2
30 4150 6
SQL>

Answer 2

Introduce a correlated subquery in select for manager name in your query.
SELECT d.department_name,
AVG(e.salary) AS "Salaries",
(SELECT first_name
||' '
||last_name
FROM employees i
WHERE i.employee_id = d.manager_id) AS manager_name
FROM employees e
join departments d
ON e.department_id = d.department_id
GROUP BY e.department_id,
d.department_name,
d.manager_id
ORDER BY Avg(e.salary);