CTE Recursive Queries - sql

Question

I have a table with records of employees that shows a relationship of employees and who they report to:
From_ID position TO_ID position
----------------------------------------
1 Lowest_employee 3 employee
3 employee 4 employee
4 employee 5 BOSS
2 Lowest_employee 6 employee
6 employee 3 employee
10 Lowest_employee 50 BOSS2
I would like to show results that look like this, with the employee / boss IDs:
EmployeeID BossID
--------------------
1 5
2 5
10 50
This means employees 1 and 2 report to ID 5 and employee 10 reports to another boss with ID 50.
I know I need to use CTE and Recursive Queries, but cannot understand how it can be done, I'm newer to CTE Recursive Queries.
I read this article but it doesn't make any sense to me MS link
Any help with the query required to achieve this would be useful.

Answer 1

This includes setting up test data, however I think this is what you want:
Test Data:
DECLARE #Table TABLE
(
From_ID int,
TO_ID int
)
INSERT INTO #Table VALUES(1,3)
INSERT INTO #Table VALUES(3,4)
INSERT INTO #Table VALUES(4,5)
INSERT INTO #Table VALUES(2,6)
INSERT INTO #Table VALUES(6,3)
INSERT INTO #Table VALUES(10,50)
Query to get answer:
;WITH Hierarchy (Employee, Superior, QueryLevel)
AS
(
--root is all employees that have no subordinates
SELECT E.From_ID, E.TO_ID, 1
FROM #Table E
LEFT
JOIN #Table S
ON S.TO_ID = E.From_ID
WHERE S.TO_ID IS NULL
--recurse up tree to final superior
UNION ALL
SELECT H.Employee, S.TO_ID, H.QueryLevel + 1
FROM Hierarchy H
JOIN #Table S
ON S.From_ID = H.Superior
)
SELECT Employee, Superior
FROM
(
SELECT *, ROW_NUMBER() OVER(PARTITION BY Employee ORDER BY QueryLevel DESC) AS RowNumber
FROM Hierarchy
) H
WHERE RowNumber = 1
Essentially, this works by :
1) get all employees with no reportees (the root)
2) recurses up through the bosses, recording the 'level'
3) use over/partition to select only the 'final' boss

Answer 2

WITH q (employee, boss) AS
(
SELECT fromId, toId
FROM mytable
WHERE fromId NOT IN
(
SELECT toId
FROM mytable
)
UNION ALL
SELECT employee, toId
FROM q
JOIN mytable t
ON t.fromId = boss
)
SELECT *
FROM q
WHERE boss NOT IN
(
SELECT fromId
FROM mytable
)

Answer 3

You could try something like this?
DECLARE #Employees TABLE (
EmployeeId INT,
PositionName VARCHAR(50),
ReportsToId INT);
INSERT INTO #Employees VALUES (1, 'Driver', 3);
INSERT INTO #Employees VALUES (3, 'Head of Driving Pool', 4);
INSERT INTO #Employees VALUES (4, 'Corporate Flunky', 5);
INSERT INTO #Employees VALUES (2, 'Window Cleaner', 6);
INSERT INTO #Employees VALUES (6, 'Head of Office Services', 3);
INSERT INTO #Employees VALUES (10, 'Minion', 50);
INSERT INTO #Employees VALUES (5, 'BOSS', NULL);
INSERT INTO #Employees VALUES (50, 'BOSS2', NULL);
WITH Employees AS (
SELECT
EmployeeId,
1 AS [Level],
EmployeeID AS [Path],
ISNULL(ReportsToId, EmployeeId) AS ReportsToId
FROM
#Employees
WHERE
ReportsToId IS NULL
UNION ALL
SELECT
e.EmployeeID,
x.[Level] + 1 AS [Level],
x.[Path] + e.EmployeeID AS [Path],
x.ReportsToId
FROM
#Employees e
INNER JOIN Employees x ON x.EmployeeID = e.ReportsToId)
SELECT
ec.EmployeeId,
e.PositionName,
ec.[Level],
CASE WHEN ec.ReportsToId = ec.EmployeeId THEN NULL ELSE ec.ReportsToId END AS ReportsToId --Can't really report to yourself
FROM
Employees ec
INNER JOIN #Employees e ON e.EmployeeId = ec.EmployeeId
ORDER BY
ec.[Path];