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.