Writing a request for the number of hired/fired each year - sql

Question

I'm trying to write an SQL request to count the number of Employees hired/fired each year.
I can have each Employee's dates with this select:
SELECT HiredDate, FiredDate FROM Employees;
I can list each year with this select:
SELECT to_char(e1.HiredDate, 'YYYY') Year FROM Employees e1
UNION
SELECT to_char(e2.FiredDate, 'YYYY') Year FROM Employees e2;
But I don't manage to count the number of hired/fired each year.
EDIT
Employees sample data:
Name | HiredDate | FiredDate
--------------------------------
John | 01/02/2003 | 03/04/2013
Jack | 05/06/2006 | 07/08/2013
Jean | 03/04/2006 | null
James | 01/02/2013 | null
Expected results:
Year | HiredNumber | FiredNumber
---------------------------------
2003 | 1 | 0
2006 | 2 | 0
2013 | 1 | 2

Answer 1

There may be years with no hiring and years with no firings. So the easiest way to solve this problem is with two sub-queries, one for each count and join them with a full outer join.
with e1 as (
select extract(year from hireddate) as emp_year
, count(hireddate) as hired_count
from employees
where hireddate is not null
group by extract(year from hireddate)
)
, e2 as (
select extract(year from fireddate) as emp_year
, count(fireddate) as fired_count
from employees
where fireddate is not null
group by extract(year from fireddate)
)
select coalesce (e1.emp_year, e2.emp_year) as emp_year
, nvl(e1.hired_count, 0) as hired_count
, nvl(e2.fired_count, 0) as fired_count
from e1
full outer join e2
on e1.emp_year = e2.emp_year
order by 1
Notes
This will exclude any years with neither hirings nor firings. It's easy enough to generate such things.
Presumably hireddate is mandatory but the not null check is retained for symmetry :)
". It works well in SQL Developer but can't be set as a Visual datasource"
Here is a variant without the FULL OUTER JOIN:
select emp_year
, sum(hired_count) as hired_count
, sum(fired_count) as fired_count
from (
select extract(year from hireddate) as emp_year
, count(hireddate) as hired_count
, 0 as fired_count
from employees
where hireddate is not null
group by extract(year from hireddate)
union all
select extract(year from fireddate) as emp_year
, 0 as hired_count
, count(fireddate) as fired_count
from employees
where fireddate is not null
group by extract(year from fireddate)
)
group by emp_year
order by 1

Answer 2

SELECT 'Hired' What, to_char(e1.HiredDate, 'YYYY') Year, COUNT(*) TheCount
FROM Employees e1
GROUP BY to_char(e1.HiredDate, 'YYYY')
UNION ALL
SELECT 'Fired' What, to_char(e2.FiredDate, 'YYYY') Year, COUNT(*) TheCount
FROM Employees e2
GROUP BY to_char(e2.FiredDate, 'YYYY');