SQL Find all direct descendants in a tree - sql

Question

I have a tree in my database that is stored using parent id links.
A sample of what I have for data in the table is:
id | name | parent id
---+-------------+-----------
0 | root | NULL
1 | Node 1 | 0
2 | Node 2 | 0
3 | Node 1.1 | 1
4 | Node 1.1.1 | 3
5 | Node 1.1.2 | 3
Now I would like to get a list of all the direct descendants of a given node but if none exist I would like to have it just return the node itself.
I want the return for the query for children of id = 3 to be:
children
--------
4
5
Then the query for the children of id = 4 to be:
children
--------
4
I can change the way I am storing the tree to a nested set but I don't see how that would make the query I want possible.

Answer 1

In new PostgreSQL 8.4 you can do it with a CTE:
WITH RECURSIVE q AS
(
SELECT h, 1 AS level, ARRAY[id] AS breadcrumb
FROM t_hierarchy h
WHERE parent = 0
UNION ALL
SELECT hi, q.level + 1 AS level, breadcrumb || id
FROM q
JOIN t_hierarchy hi
ON hi.parent = (q.h).id
)
SELECT REPEAT(' ', level) || (q.h).id,
(q.h).parent,
(q.h).value,
level,
breadcrumb::VARCHAR AS path
FROM q
ORDER BY
breadcrumb
See this article in my blog for details:
PostgreSQL 8.4: preserving order for hierarchical query
In 8.3 or earlier, you'll have to write a function:
CREATE TYPE tp_hierarchy AS (node t_hierarchy, level INT);
CREATE OR REPLACE FUNCTION fn_hierarchy_connect_by(INT, INT)
RETURNS SETOF tp_hierarchy
AS
$$
SELECT CASE
WHEN node = 1 THEN
(t_hierarchy, $2)::tp_hierarchy
ELSE
fn_hierarchy_connect_by((q.t_hierarchy).id, $2 + 1)
END
FROM (
SELECT t_hierarchy, node
FROM (
SELECT 1 AS node
UNION ALL
SELECT 2
) nodes,
t_hierarchy
WHERE parent = $1
ORDER BY
id, node
) q;
$$
LANGUAGE 'sql';
and select from this function:
SELECT *
FROM fn_hierarchy_connect_by(4, 1)
The first parameter is the root id, the second should be 1.
See this article in my blog for more detail:
Hierarchical queries in PostgreSQL
Update:
To show only the first level children, or the node itself if the children do not exist, issue this query:
SELECT *
FROM t_hierarchy
WHERE parent = #start
UNION ALL
SELECT *
FROM t_hierarchy
WHERE id = #start
AND NOT EXISTS
(
SELECT NULL
FROM t_hierarchy
WHERE parent = #start
)
This is more efficient than a JOIN, since the second query will take but two index scans at most: the first one to make sure to find out if a child exists, the second one to select the parent row if no children exist.

Answer 2

Found a query that works the way I wanted.
SELECT * FROM
( SELECT id FROM t_tree WHERE name = '' ) AS i,
t_tree g
WHERE
( ( i.id = g.id ) AND
NOT EXISTS ( SELECT * FROM t_tree WHERE parentid = i.id ) ) OR
( ( i.id = g.parentid ) AND
EXISTS ( SELECT * FROM t_tree WHERE parentid = i.id ) )