One Level At A Time

employees:

id	manager_id
1	NULL
2	1
3	1
4	2
5	2
6	4

WITH RECURSIVE chain(id, depth) AS (
    SELECT id, 0
    FROM employees
    WHERE manager_id IS NULL
  UNION ALL
    SELECT e.id, c.depth + 1
    FROM employees e
    JOIN chain c ON e.manager_id = c.id
)
SELECT depth, count(*) FROM chain GROUP BY depth ORDER BY depth;

What is the output?

0,6

0,1 1,2 2,2 3,1

0,1 1,2 2,3

0,1 1,5