Recursive CTE not getting desired result. How to assign anchor only? - sql

Question

The application is for an employee id get all managers
declare #emp table (id int primary key, mgr int);
insert into #emp values
(1, null)
, (2, 1)
, (3, 2)
, (4, null)
, (5, 4);
select * from #emp;
; with cte as
( select e.id, e.mgr, cnt = 1
from #emp e
union all
select e.id, e.mgr, cnt + 1
from #emp e
join cte
on cte.mgr = e.id
)
select id, mgr, cnt
from cte
where id = 3;
The above only returns the single row for id = 3. I get that is expected but not what I desire. I want to start (anchor) at 3 and get the manager chain.
If I hard code the anchor I get the desired result.
See below:
; with cte as
( select e.id, e.mgr, cnt = 1
from #emp e
where e.id = 3
union all
select e.id, e.mgr, cnt + 1
from #emp e
join cte
on cte.mgr = e.id
)
select id, mgr, cnt
from cte;
My question is how to assign the anchor (top part) only in a where on the cte?
If not in the where is there another way to assign the anchor only (not hard coding in the cte)?

Answer 1

You need to keep the starting position in your cte:
declare #emp table (id int primary key, mgr int);
insert into #emp values
(1, null)
, (2, 1)
, (3, 2)
, (4, null)
, (5, 4);
; with cte as
( select e.id ori, e.id, e.mgr, cnt = 1
from #emp e
union all
select cte.ori, e.id, e.mgr, cnt + 1
from #emp e
join cte
on cte.mgr = e.id
)
select ori, id, mgr, cnt
from cte
where cte.ori = 3;
Result:
+-----+----+------+-----+
| ori | id | mgr | cnt |
+-----+----+------+-----+
| 3 | 3 | 2 | 1 |
| 3 | 2 | 1 | 2 |
| 3 | 1 | NULL | 3 |
+-----+----+------+-----+

Answer 2

I made just one change and removed (seemingly) unnecessary column:
declare #emp table (id int primary key, mgr int);
insert into #emp values
(1, null)
, (2, 1)
, (3, 2)
, (4, null)
, (5, 4);
select * from #emp;
; with cte as
( select e.id, e.mgr
from #emp e
union all
select cte.id, e.mgr
from #emp e
join cte
on cte.mgr = e.id
)
select id, mgr
from cte
where id = 3;
Result is:
id | mgr
3 | 2
3 | 1
3 | NULL

Answer 3

You must decide whether to
start with a root node (add WHERE mgr IS NULL or an explicit id to your anchor) and move down the chain, or to
start with any child node (add where not exists(SELECT 1 FROM #emp AS x WHERE e.id=x.mgr) to the anchor) and move up or to
start with an explicit child (add where e.id=3 to the anchor) and move up.
The general advise is: Start with the narrowest anchor-set possible!
The first query you state in your question (as well as other answers here) will do a huge overdose creating each an any chain starting from everywhere with overlapping results.
As the recursive CTE is a hidden RBAR the engine has no chance to predict its result and will create the full load - just to throw most of this away.
If this is a 1:n relation (always 1 mgr on-top) moving upwards will be much faster. Starting from a given child node You have exactly one step per level - that's it.
Applying the filter in a WHERE after the recursice CTE was worked out, means to create any possible chain just to throw most of them away...
Your second approach is the best I can think of actually.
So the question is: Why do you want to apply this filter at the end?