在同一SQL查询中使用ROUND，AVG和COUNT

Question

I need to write a query where I need to first count the people working in a department, then calculate the average people working in a department and finally round it to only one decimal place. I tried so many different variations.

That's what I got so far although it's not the first one I tried but I always get the same error message. (ORA-00979 - not a group by expression)

SELECT department_id,
    ROUND(AVG(c.cnumber),1)
FROM employees c
 WHERE c.cnumber =
           (SELECT COUNT(c.employee_id)
                FROM employees c)
GROUP BY department_id;

I really don't know what do to at this point and would appreciate any help.

Employees Table:

Answer 1

Try this (Oracle syntax) example from your description:

with department_count as (
SELECT department_id, COUNT(c.employee_id) as employee_count
FROM employees c
group by department_id
)
SELECT department_id,
    ROUND(AVG(c.employee_count),1)
FROM department_count c
GROUP BY department_id;

But this query not make sense. Count is integer, and count return one number for one department in this case AVG return the same value as count.

Maybe you have calculate number of employee and averange of salary on department?