Group By Month between two dates - sql

Question

I have a table that lists all employees and their respective start and end dates
I want to be able to count the number of active employees in each month. Is there a way to do this via a single query (eg groupBy) rather than generating multiple queries for each month?
=================================
Employee ID | StartDt | EndDt
123 | 01 Feb 2022 |
234 | 01 jan 2022 | 28 Feb 2022
456 | 01 dec 2021 | 28 Feb 2022
As an example, the table above should return:
Dec 2021: 1
Jan 2022: 2
Feb 2022: 3
Mar 2022: 1
Apr 2022: 1

Answer 1

You can generate a calendar and join to that:
WITH calendar (month) AS (
SELECT ADD_MONTHS(min_startdt, LEVEL - 1)
FROM (
SELECT MIN(startdt) AS min_startdt,
MAX(COALESCE(enddt, SYSDATE)) AS max_enddt
FROM employees
)
CONNECT BY LEVEL - 1 <= MONTHS_BETWEEN(max_enddt, min_startdt)
)
SELECT c.month,
COUNT(e.employee_id)
FROM calendar c
LEFT OUTER JOIN employees e
ON (e.startdt <= c.month AND (c.month <= e.enddt OR e.enddt IS NULL))
GROUP BY
c.month
Which, for the sample data:
CREATE TABLE employees (Employee_ID, StartDt, EndDt ) AS
SELECT 123, DATE '2022-02-01', NULL FROM DUAL UNION ALL
SELECT 234, DATE '2022-01-01', DATE '2022-02-28' FROM DUAL UNION ALL
SELECT 456, DATE '2021-12-01', DATE '2022-02-28' FROM DUAL;
Outputs:
MONTH
COUNT(E.EMPLOYEE_ID)
2021-12-01 00:00:00
1
2022-01-01 00:00:00
2
2022-02-01 00:00:00
3
2022-03-01 00:00:00
1
2022-04-01 00:00:00
1
db<>fiddle here