How to use count() without the output changing when I use join -Postgresql - sql

Question

I need to find the sum of all the rental days of a car dealership (including
period_begin and period_end, not unique) for all cars of that department.Divided by the total number of different (unique) employees that department ever had.
I have 5 tables
Department(PK departmentnr, name, FK postcode, FK place_name)
Employee(PK employeenr, FK email)
Contract(PK periode_begin, PK periode_end, FK departmentnr, FK employeenr)
registerform (,PK periode_end,PK,periode_end,FKemail,FK,
Fk numberplate,periode_end,periode_begin)
car(PK numberplate,FK departmentnr,FK Brand,FK model)
when I go step by step
part 1
The total employees per department
select departmentnr,Count(employeenr)FROM contract
group by departmentnr
part 2
the amount of days the cars were hired
SELECT DISTINCT departmentnr,
Sum((( Date(periode_end) - Date(periode_begin) + 1 ))) AS
average
FROM registerform r
INNER JOIN car w using(numberplate)
GROUP BY departmentnr
I get the correct ouput but when I try to get these 2 together
SELECT distinct departmentnr,
(
sum(((date(r.periode_end) - date(r.periode_begin) + 1))) / (
select
count(employeenr))
)
as average
from
registerform r
inner join
car w using(numberplate)
inner join
contract using(departmentnr)
inner join
employee using(employeenr)
group by
departmentnr
then my output gets absurd.
How can I fix this and is there a way to make the code more efficient.

Answer 1

Aggregated before you JOIN. So, one method is:
SELECT c.departmentnr, co.num_employees,
Sum( Date(r.periode_end) - Date(r.periode_begin) + 1 ) AS average
FROM registerform r JOIN
car c
USING (numberplate) LEFT JOIN
(SELECT co.departmentnr, Count(*) as num_employees
FROM contract co
GROUP BY co.departmentnr
) co
ON co.departmentnr = c.departmentnr
GROUP BY c.departmentnr, co.num_employees;