How to get the parent given a child in SQL SERVER 2005 - sql-server-2005

Question

I have a table like this
childid parentid
------------------------
1 0
2 1
3 2
4 2
5 3
6 4
7 0
8 7
9 8
10 1
If I give a childid as 5, the parentid will be 1(output)
If I give a childid as 9, the parentid will be 7.(output)
i.e. the root parentid is 0 and the query should stop there.
How to solve such a query?
Please help.

Answer 1

I think you should rename your child_id to node, your parent_id to child_of. Your column naming is a bit confusing
create table stack_overflow
(
node int, child_of int
);
insert into stack_overflow(node, child_of) values
(1,0),
(2,1),
(3,2),
(4,2),
(5,3),
(6,4),
(7,0),
(8,7),
(9,8),
(10,1);
This works on any CTE-capable RDBMS:
with find_parent(parent, child_of, recentness) as
(
select node, child_of, 0
from stack_overflow
where node = 9
union all
select i.node, i.child_of, fp.recentness + 1
from stack_overflow i
join find_parent fp on i.node = fp.child_of
)
select top 1 parent from find_parent
order by recentness desc
Output:
parent
7
[EDIT: more flexible and future-proof]:
with find_parent(node_group, parent, child_of, recentness) as
(
select node, node, child_of, 0
from stack_overflow
where node in (5,9)
union all
select fp.node_group, i.node, i.child_of, fp.recentness + 1
from stack_overflow i
join find_parent fp on i.node = fp.child_of
)
select q.node_group as to_find, parent as found
from find_parent q
join
(
select node_group, max(recentness) as answer
from find_parent
group by node_group
) as ans on q.node_group = ans.node_group and q.recentness = ans.answer
order by to_find
Output:
to_find found
5 1
9 7
If you're using Postgres, the above code could be shortened to:
with recursive find_parent(node_group, parent, child_of, recentness) as
(
select node, node, child_of, 0
from stack_overflow
where node in (5,9)
union all
select fp.node_group, i.node, i.child_of, fp.recentness + 1
from stack_overflow i
join find_parent fp on i.node = fp.child_of
)
select distinct on (node_group) node_group as to_find, parent as found
from find_parent
order by to_find, recentness desc
DISTINCT ON rocks! :-)

Answer 2

If ALL you want is the root ParentID, you can use this recursive function:
CREATE FUNCTION test_func
(
#ParentID int
)
RETURNS int
AS
BEGIN
DECLARE #result int;
DECLARE #childID int;
SET #childID = (SELECT ChildID FROM YourTable WHERE ParentID = #ParentID)
IF (#childID = 0)
SET #result = #ParentID
ELSE
SET #result = dbo.test_func(#childID)
RETURN #result
END
GO
then in your main query:
SELECT dbo.test_func(5)
Passing in 5 returns 1, 9 returns 7 based on your provided data. If you need every ParentID that is up that chain, you should probably use a CTE.

Answer 3

I think you want a recursive query, you should use Common Table Expressions. I will give you a link with an example very similar that the one you're using.
I think here is the solution. It helped me some months ago.

Answer 4

A simple of example of getting the parent ID matching a given child ID is:
select parentid
from MyTable
where childid = 5
However, for the data above, this will return no records.