Postgresql – CTE: Get all parents and all children in one statement

ctepostgresqlrecursive

I have this working CTE example.

I can select all grand-parents and all children.

But how can I select all grand-parents and all children in one statement?

In this example I want Grandfather, Father, Son as output if I give "Father" as input.

I use PostgreSQL. But I think this question should be standard SQL.

Please correct me if I use PostgreSQL specific syntax.

DROP table if exists tree;

CREATE TABLE tree (
 id SERIAL PRIMARY KEY,
 name character varying(64) NOT NULL,
 parent_id integer REFERENCES tree NULL
);

insert into tree values (1, 'Grandfather', NULL);
insert into tree values (2, 'Father', 1);
insert into tree values (3, 'Son', 2);


-- -------------------------------------
-- Getting all children works  

WITH RECURSIVE rec (id) as
(
  SELECT tree.id, tree.name from tree where name='Father'

  UNION ALL

  SELECT tree.id, tree.name from rec, tree where tree.parent_id = rec.id



  )
SELECT *
FROM rec;

-- Result: 
--  id |  name  
-- ----+--------
--   2 | Father
--   3 | Son



-- -------------------------------------
-- Getting all parents works

WITH RECURSIVE rec (id) as
(
  SELECT tree.id, tree.name, tree.parent_id from tree where name='Father'

  UNION ALL

  SELECT tree.id, tree.name, tree.parent_id from rec, tree where tree.id = rec.parent_id
  )
SELECT id, name
FROM rec;

-- Result
-- id |    name     
-- ----+-------------
--  2 | Father
--  1 | Grandfather

Update

Above is a simplified working example. The tree can be up to 100 level deep. There can be several level of ancestors above "Father" and several level of descendants below. I want all ancestors and all descendants.

Best Answer

If you want all ancestors and all descendants, you can combine the two queries in one. Use the two CTEs and then a simple UNION:

WITH RECURSIVE 
    -- descendants 
    rec_d (id, name) AS
    (
      SELECT tree.id, tree.name FROM tree WHERE name = 'Father'
      UNION ALL
      SELECT tree.id, tree.name FROM rec_d, tree where tree.parent_id = rec_d.id
    ),
    --  ancestors
    rec_a (id, name, parent_id) AS
    (
      SELECT tree.id, tree.name, tree.parent_id FROM tree WHERE name = 'Father'
      UNION ALL
      SELECT tree.id, tree.name, tree.parent_id FROM rec_a, tree WHERE tree.id = rec_a.parent_id
    )
SELECT id, name FROM rec_a
UNION 
SELECT id, name FROM rec_d ;

If we haven't got any errors above, we can them improve it:

  • change the final UNION to UNION ALL by putting the starting node(s) in only one of the CTEs.
  • use JOIN .. ON instead of the implicit joins.
  • fix mismatches between SELECT and CTE column lists.

The query becomes:

WITH RECURSIVE 
    -- starting node(s)
    starting (id, name, parent_id) AS
    (
      SELECT t.id, t.name, t.parent_id
      FROM tree AS t
      WHERE t.name = 'Father'          -- this can be arbitrary
    ),
    descendants (id, name, parent_id) AS
    (
      SELECT s.id, s.name, s.parent_id 
      FROM starting AS s
      UNION ALL
      SELECT t.id, t.name, t.parent_id 
      FROM tree AS t JOIN descendants AS d ON t.parent_id = d.id
    ),
    ancestors (id, name, parent_id) AS
    (
      SELECT t.id, t.name, t.parent_id 
      FROM tree AS t 
      WHERE t.id IN (SELECT parent_id FROM starting)
      UNION ALL
      SELECT t.id, t.name, t.parent_id 
      FROM tree AS t JOIN ancestors AS a ON t.id = a.parent_id
    )
TABLE ancestors
UNION ALL
TABLE descendants ;
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