Find Parent Recursively using Query

Question

I am using postgresql. I have the table as like below

parent_id    child_id
----------------------
101       102
103       104
104       105
105       106            


        

Answer 1

Use WITH RECURSIVE to create a Common Table Expression (CTE). For the non-recursive term, get the rows in which the child is immediately below the parent:

SELECT
   c.child_id,
   c.parent_id
FROM
   mytable c
LEFT JOIN
   mytable p ON c.parent_id = p.child_id
WHERE
   p.child_id IS NULL

 child_id | parent_id
----------+-----------
      102 |       101
      104 |       103

For the recursive term, you want the children of these children.

WITH RECURSIVE tree(child, root) AS (
   SELECT
      c.child_id,
      c.parent_id
   FROM
      mytable c
   LEFT JOIN
      mytable p ON c.parent_id = p.child_id
   WHERE
      p.child_id IS NULL
   UNION
   SELECT
      child_id,
      root
   FROM
      tree
   INNER JOIN
      mytable on tree.child = mytable.parent_id
)
SELECT * FROM tree;

 child | root
-------+------
   102 |  101
   104 |  103
   105 |  103
   106 |  103

You can filter the children when querying the CTE:

WITH RECURSIVE tree(child, root) AS (...) SELECT root FROM tree WHERE child = 106;

 root
------
  103