[SQL] Linked list with CTE

[Mark Lubratt]

Hello,

I have a table in my database with multiple, independent linked lists.  I would 
like to have a query that returns an entire linked list given a node (the node 
could be anywhere within the list).

I found on the web an example of how to use CTEs to do this:

http://wiki.postgresql.org/wiki/CTEReadme

I'll repeat the gist of it here:

CREATE TABLE department (
    id INTEGER PRIMARY KEY,  -- department ID
    parent_department INTEGER REFERENCES department, -- upper department ID
    name TEXT -- department name
);

INSERT INTO department (id, parent_department, "name")
VALUES
     (0, NULL, 'ROOT'),
     (1, 0, 'A'),
     (2, 1, 'B'),
     (3, 2, 'C'),
     (4, 2, 'D'),
     (5, 0, 'E'),
     (6, 4, 'F'),
     (7, 5, 'G');

-- department structure represented here is as follows:
--
-- ROOT-+->A-+->B-+->C
--      |         |
--      |         +->D-+->F
--      +->E-+->G
To extract all departments under A, you can use the following recursive query:
WITH RECURSIVE subdepartment AS
(
    -- non-recursive term
    SELECT * FROM department WHERE name = 'A'

    UNION ALL

    -- recursive term
    SELECT d.*
    FROM
        department AS d
    JOIN
        subdepartment AS sd
        ON (d.parent_department = sd.id)
)
SELECT *
FROM subdepartment
ORDER BY name;

My database contains multiple, independent structures like the one given above. 
 So, I can modify the above with:

insert into department (id, parent_department, name) values (8, NULL, 'Z'), (9, 
8, 'Y');

I need a bidirectional query and since I'm quite new to CTE, I'm not sure how 
to modify the query to get parent departments as well as subdepartments...  
Thus, if I give the query any node in a linked list, I'd like the entire tree 
returned.

e.g.  If I give the query 'A', I'd like it to return the ROOT, A, B, C, D, E, 
F, G tree.  If I give the query 'Y', I'd like it to return the Z, Y tree.

I hope I made sense...

Thanks!
Mark