Counting the hierarchy - sql

Question

I have a table with a hierarchy:
CREATE TABLE mng
(
id INTEGER NOT NULL PRIMARY KEY,
name VARCHAR(50) NOT NULL,
manager_id INTEGER
);
INSERT INTO mng (id, name, manager_id) VALUES (1, 'Lola', NULL);
INSERT INTO mng (id, name, manager_id) VALUES (2, 'Bella', NULL);
INSERT INTO mng (id, name, manager_id) VALUES (3, 'Lo', 1);
INSERT INTO mng (id, name, manager_id) VALUES (4, 'Ann', 2);
INSERT INTO mng (id, name, manager_id) VALUES (5, 'Ki', 3);
INSERT INTO mng (id, name, manager_id) VALUES (6, 'Qo', 5);
I need to print all top managers (where manager_id is NULL) with their subordinates count (including all levels)
id | cnt
--------+---------
1 | 3
2 | 1
upd
was trying something like this:
WITH DirectReports(ManagerID, Employee) AS
(
SELECT id, 0 AS Employee
FROM mng
WHERE manager_id IS NULL
UNION ALL
SELECT e.id, Employee + 1
FROM mng AS e
INNER JOIN DirectReports AS d
ON e.manager_id = d.ManagerID
)
SELECT ManagerID, Employee
FROM DirectReports
ORDER BY ManagerID;
got level number, but how to get count?

Answer 1

The issue with your original code is you lose track of the "top manager". You can, however, retain this within the CTE, and simply have it listed for every employee:
WITH DirectReports(id, TopMgr, ManagerID, Employee) AS
(
SELECT id, id, NULL, 0 AS Employee
FROM mng
WHERE manager_id IS NULL
UNION ALL
SELECT e.id, d.TopMgr, e.manager_id, 1 AS Employee
FROM mng AS e
INNER JOIN DirectReports AS d
ON e.manager_id = d.id
)
SELECT TopMgr, SUM(Employee)
FROM DirectReports
GROUP BY TopMgr
ORDER BY TopMgr
Output:
TopMgr EmployeeCount
1 3
2 1

Answer 2

First you can get the employees under manager, top level manager. Later, you can count the employees under top level manager.
;WITH DirectReports(EmployeeId, ManagerID, TopMostManagerId) AS
(
SELECT id, Null, id
FROM #mng
WHERE manager_id IS NULL
UNION ALL
SELECT e.id, e.manager_Id, d.TopMostManagerId
FROM #mng AS e
INNER JOIN DirectReports AS d
ON e.manager_id = d.EmployeeId
)
SELECT TopMostManagerId, COUNT(EmployeeId) as CountOfEmployees
FROM DirectReports
WHERE ManagerID is not null -- excludes toplevel manager
group by TopMostManagerId
TopMostManagerId
CountOfEmployees
1
3
2
1