Help with store procedure - sql-server

Question

I need help with the following sp. Couldnt solve it really.
I have 2 columns in a table:
1. Parent
2. Child
I need to create a sp using rownum that will create a new table with a given param (level num) that will display the connection between the parent and son, or grandfather & grandson, etc...
For instance:
Parent | Child
2 4
4 6
the sp will return with a given level of 1 the same table, but if I hand it level 2 (grandfather<-> grandson) it will show:
Parent | Child
2 6
How can I do that?
Thanks all!

Answer 1

Here is some code using a recursive cte that does what I believe it is you want.
-- Sample data
declare #T table (ID int, ParentID int)
insert into #T values
(1, null),
(2, 1),
(3, 1),
(4, 3),
(5, 3),
(6, 1),
(7, null),
(8, 7),
(9, 8),
(10, 9)
-- The level you want
declare #Level int = 2
-- recursive cte
;with cte as
(
select
ID,
ParentID,
0 as lvl
from #T
where ParentID is null
union all
select
T.ID,
T.ParentID,
C.lvl+1 as lvl
from #T as T
inner join cte as C
on T.ParentID = C.ID
where C.lvl < #Level
)
select *
from cte
where lvl = #Level

Answer 2

DECLARE #level INT
SET #level = 3
DECLARE #temp TABLE
(
parent INT,
child INT
)
DECLARE #result TABLE
(
parent INT,
child INT
)
INSERT INTO #result
SELECT * FROM testing
WHILE #level > 1
BEGIN
SET #level = #level - 1
DELETE #temp
INSERT INTO #temp
SELECT t.parent, r.child
FROM testing t
INNER JOIN #result r
ON t.child = r.parent
DELETE #result
INSERT INTO #result
SELECT * FROM #temp
END
SELECT * FROM #result

Answer 3

You can use auto join. For level 2:
SELECT
p.Parent
,c.Child
FROM
Table_1 p
JOIN Table_1 c
ON p.Child = c.Parent
level 3:
SELECT
g.Parent
,c.Child
FROM
Table_1 g
JOIN Table_1 p
ON g.Child = p.Parent
JOIN Table_1 c
ON p.Child = c.Parent
If you need solution for arbitrary level, try to generate dinamic sql based on level.
CREATE PROCEDURE GetConnected
#level INT
AS
BEGIN
DECLARE #sql VARCHAR(1000)
SET #sql = 'SELECT t0.Parent, t' + CAST(#level AS VARCHAR(5)) + '.Child FROM Table_1 t0 '
DECLARE #counter INT
SET #counter = 0
WHILE #counter < #level
BEGIN
SET #sql = #sql + 'JOIN Table_1 t' + CAST(#counter + 1 AS VARCHAR(5)) + ' ON t' + CAST(#counter AS VARCHAR(5)) + '.Child = t' + CAST(#counter + 1 AS VARCHAR(5)) + '.Parent '
SET #counter = #counter + 1
END
EXEC (#sql)
END

Answer 4

This solution, for which I've shamelessly borrowed Mikael's sample data definition, shows all the relations represented, building pairs not only from the top level (parentless) items, but also from the lower level ones. The author of the question didn't specify whether it should be one way or the other.
/* Sample data */
DECLARE #T TABLE (ID int, ParentID int);
INSERT INTO #T VALUES
(1, NULL),
(2, 1),
(3, 1),
(4, 3),
(5, 3),
(6, 1),
(7, NULL),
(8, 7),
(9, 8),
(10, 9);
/* The level you want */
DECLARE #Level int;
SET #Level = 2;
/* The query */
WITH recur AS (
SELECT
ID,
AncestorID = ParentID,
Level = CASE WHEN ParentID IS NULL THEN 0 ELSE 1 END
FROM #T
UNION ALL
SELECT
r.ID,
t.AncestorID,
Level = r.Level + 1
FROM #T t
INNER JOIN recur r ON t.ID = r.AncestorID
WHERE r.Level < #Level
)
SELECT ID, ParentID
FROM recur
WHERE Level = #Level
AND (#Level = 0 OR ParentID IS NOT NULL)
For the given #Level value of 2 the result will be:
ID AncestorID Level
----------- ----------- -----------
10 8 2
9 7 2
5 1 2
4 1 2