CODEWITHSUNDEEP
sqlhierarchical-datarecursive-query
therko
therko

Reputation: 69

SQL Get lowest level child and root node

I have database schema: [Id], [ParrentId], [some more tables]

I have hierarchy like:

1. a
 2. aa
  3. aaa_1
  3. aaa_2
1. b
 2. bb
1. c
 2. cc
  3. ccc_1
   4. cccc
  3. ccc_2

I want a (select * where X) => [X, lowest leve child] like: [a, aaa_1] [a, aaa_2]; [cc, cccc] etc.

I can get lowest child with 

SELECT t1.name FROM
category AS t1 LEFT JOIN category as t2
ON t1.category_id = t2.parent
WHERE t2.category_id IS NULL;

but I don't know how to join it with root node.

Upvotes: 0

Views: 4313

Answers (2)

S. Rojak
S. Rojak

Reputation: 454

Given:

  • The DBMS is SQL Server;
  • The highest level nodes of the tree have parent = NULL;
  • You want all the lowest leaves for all levels of the trees, not just the roots;
  • You want to have all the nodes at a lowest level, not just one;

This query would do it:

WITH r ( category_id, name, root, depth )
-- finds the root relationship
AS (
    SELECT category_id, name, category_id, 0
        FROM category
        -- WHERE parent IS NULL -- this would only look at root nodes
    UNION ALL
    SELECT c.category_id, c.name, r.root, r.depth + 1
        FROM r
        JOIN category c
            ON c.parent = r.category_id
), s ( category_id, name, root, window_id )
-- finds the lowest leaves
AS (
    SELECT category_id, name, root, RANK() OVER(partition by root order by depth DESC)
        FROM r
)
SELECT c.name AS NodeName, s.Name AS DeepLeafName
    FROM category c
    JOIN s
        ON c.category_id = s.root
        WHERE s.window_id = 1;

Here is the result set:

result set

Upvotes: 1

Polux2
Polux2

Reputation: 592

With SQL Server, you can try this :

With CTE as 
(
Select ID as Child, lev = 1
from category
where ID = X
UNION ALL
Select category.ID, CTE.lev + 1
from category
    inner join CTE ON category.ParentID = CTE.Child
)

select CTE_1.Child, CTE_2.Child 
from CTE as CTE_1 
    inner join CTE as CTE_2
where CTE_1.lev  = 1 AND CTE_2.lev = (select MAX(CTE.lev) from CTE)

Upvotes: 0

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