How to develop a recursive CTE in T-SQL? - sql-server

Question

I am new to recursive CTEs. I am trying to develop a CTE which will return all of the employees under each manager name. So I have two tables: people_rv and staff_rv
People_rv table contains all of the people, both managers and employees. Staff_rv only contains manager information. Uniqueidentifier staff values are stored in Staff_rv. Uniqueidentifier employee values are stored in people_rv. People_rv contains varchar first and last name values for both managers and employees.
But when I run the following CTE I get an error:
WITH
cteStaff (ClientID, FirstName, LastName, SupervisorID, EmpLevel)
AS
(
SELECT p.people_id, p.first_name, p.last_name, s.supervisor_id,1
FROM people_rv p JOIN staff_rv s on s.people_id = p.people_id
WHERE s.supervisor_id = '95E16819-8C3A-4098-9430-08F0E3B764E1'
UNION ALL
SELECT p2.people_id, p2.first_name, p2.last_name, s2.supervisor_id, r.EmpLevel + 1
FROM people_rv p2 JOIN staff_rv s2 on s2.people_id = p2.people_id
INNER JOIN cteStaff r on s2.staff_id = r.ClientID
)
SELECT
FirstName + ' ' + LastName AS FullName,
EmpLevel,
(SELECT first_name + ' ' + last_name FROM people_rv p join staff_rv s on s.people_id = p.people_id
WHERE s.staff_id = cteStaff.SupervisorID) AS Manager
FROM cteStaff
OPTION (MAXRECURSION 0);
My output is:
Barbara G 1 Melanie K
Dawn P 1 Melanie K
Garrett M 1 Melanie K
Stephanie P 1 Melanie K
Amanda F 1 Melanie K
Amanda T 1 Melanie K
Stephanie G 1 Melanie K
Carlos H 1 Melanie K
So it is not iterating any more than the first level. Why not?
Melanie is the top most supervisor, but each of the persons in the leftmost column are also supervisors. So this query should also return level 2.

Answer 1

You may be in an infinite loop with your join. I would check how many levels you expect the table to actually go down. Generally you join a recursion on something similar to do
ID = ParentID
of something either contained in a table or in an expression. Keep in mind you can also create a CTE prior to a recursive CTE if you have to make up your relationship.
Here is an example that will self execute, it may help.
Declare #table table ( PersonId int identity, PersonName varchar(512), Account int, ParentId int, Orders int);
insert into #Table values ('Brett', 1, NULL, 1000),('John', 1, 1, 100),('James', 1, 1, 200),('Beth', 1, 2, 300),('John2', 2, 4, 400);
select
PersonID
, PersonName
, Account
, ParentID
from #Table
; with recursion as
(
select
t1.PersonID
, t1.PersonName
, t1.Account
--, t1.ParentID
, cast(isnull(t2.PersonName, '')
+ Case when t2.PersonName is not null then '\' + t1.PersonName else t1.PersonName end
as varchar(255)) as fullheirarchy
, 1 as pos
, cast(t1.orders +
isnull(t2.orders,0) -- if the parent has no orders than zero
as int) as Orders
from #Table t1
left join #Table t2 on t1.ParentId = t2.PersonId
union all
select
t.PersonID
, t.PersonName
, t.Account
--, t.ParentID
, cast(r.fullheirarchy + '\' + t.PersonName as varchar(255))
, pos + 1 -- increases
, r.orders + t.orders
from #Table t
join recursion r on t.ParentId = r.PersonId
)
, b as
(
select *, max(pos) over(partition by PersonID) as maxrec -- I find the maximum occurrence of position by person
from recursion
)
select *
from b
where pos = maxrec -- finds the furthest down tree
-- and Account = 2 -- I could find just someone from a different department

Answer 2

Your problem as far as I can tell is is you have no join connecting managers to their employees.
This join
INNER JOIN cteStaff r on r.StaffID = s2.staff_id
Just joins the same initial level 1 staffer back to himself.
UPDATE:
Still not quite right! You have a supervisor_id, but again you're still not actually using that to join back to the CTE.
So for each recursion of this CTE you need to (excluding the name join):
select {Level 1 Boss}, NULL (no supervisor)
union
select {new employee}, {that employee's boss}
So the join must connect the CTE's ClientID (the level 1 boss) to the second UNION query's supervisor field, which looks to be supervisor_id , not staff_id.
The JOIN to accomplish this second task is (from what I can tell of your staff_rv table schema):
SELECT p2.people_id, p2.first_name, p2.last_name, s2.supervisor_id, r.EmpLevel + 1
FROM people_rv p2 JOIN staff_rv s2 on s2.people_id = p2.people_id
INNER JOIN cteStaff r on s2.supervisor_id = r.ClientID
Note the bottom join joins the r.ClientID (the level 1 boss) to the staffer's supervisor_id field.
(NB: I think your staff_id and supervisor_id's mimic your people_id values from the people_rv table, so this join should work fine. But if they are different (i.e. a staffer's supervisor_id isn't that supervisor's people_id) then you'll need to write the join such that the staffer's supervisor_id can be joined to their people_id you're storing as ClientID in the CTE.)

Answer 3

Here's a good simple Recursive CTE to review (it may not be the answer, but someone else searching on how to make a recursive CTE may need it):
-- Recursive CTE
;
WITH Years ( myYear )
AS (
-- Base case
SELECT DATEPART(year, GETDATE())
UNION ALL
-- Recursive
SELECT Years.myYear - 1
FROM Years
WHERE Years.myYear >= 2002
)
SELECT *
FROM Years

Answer 4

Note that this probably won't solve your problem, but is a means to hopefully seeing where you're going wrong in the original query.
The default is 100 levels of recursion - you can set it to unlimited by using the MAXRECURSION query hint where you're selecting from your CTE:
...
FROM cteStaff
OPTION (MAXRECURSION 0);
From MSDN:
MAXRECURSION number
Specifies the maximum number of recursions allowed for this query. number is a nonnegative integer between 0 and 32767. When 0 is
specified, no limit is applied. If this option is not specified, the
default limit for the server is 100.
When the specified or default number for MAXRECURSION limit is reached during query execution, the query is ended and an error is
returned.
Because of this error, all effects of the statement are rolled back. If the statement is a SELECT statement, partial results or no
results may be returned. Any partial results returned may not include
all rows on recursion levels beyond the specified maximum recursion
level.