How to retrieve a child node's immediate parent node from a tree table?

Question

Hello
Hi,
I have a category_subcategories table, and I would like to know how to construct a sql and sub-sql for retrieving a child node's immediate parent node.
Here is my first part of the sql, it only returns the node "Flash"'s parent and its grand-parents:
SELECT parent.category_name, node.lft, node.rgt
FROM category_subcategories AS node,
category_subcategories AS parent
WHERE node.lft > parent.lft AND node.lft < parent.rgt
AND node.category_name = 'FLASH'
ORDER BY parent.lft;
| name |
| ELECTRONICS |
| PORTABLE ELECTRONICS |
| MP3 PLAYERS | |
how can I modify this query so that it returns Flash' parent - 'MP3 Players'?
Thanks a lot
Sam

Answer 1

Hi,
This is an Oracle forum. If you're not iusing Oracle, make that clear. Always say what version of your softwate you're using, whether it's Oracle or anything else.
Whenever you have a question, post a little sample data (CREATE TABLE and INSERT statements), and the results you want from that data. Explain how you get those results from that data.
It looks like you're using the Nested Sets technique for modeling a tree. To get the parents of given nodes, do something like this:
SELECT        parent.category_name
,       node.lft
,       node.rgt
FROM        category_subcategories      node     -- Can't use AS with table alias in Oracle
,       category_subcategories      parent
WHERE        parent.lft      IN (
                    SELECT     MAX (lft)
                    FROM     category_subcategories
                    WHERE     lft     < node.lft
                    AND     rgt     > node.rgt
AND        node.category_name          = 'FLASH'
ORDER BY  parent.lft; This should work in Oracle 8.1 and up. (I can't actually test it unless you post CREATE TABLE and INSERT statements for some sample data). You may need to modify the syntax a little for your database.
785102 wrote:
Hello,
I tried to implement the solution as follow:
mysql> select parent.*
-> from category_subcategories as parent
-> having parent.lft =
-> (select max(parent.lft) from
-> (SELECT parent.category_name, parent.lft, parent.rgt
-> FROM category_subcategories AS node,
-> category_subcategories AS parent
-> WHERE node.lft > parent.lft AND node.lft < parent.rgt
-> AND node.category_name = 'Sofa'
-> ORDER BY parent.lft
-> )
-> );
ERROR 1248 (42000): Every derived table must have its own alias
mysql>
But I got an error.
What is wrong with it?What does the error message say?
Apparantly, in your system (unlike Oracle), every sub-query must have a name. Try something like this:
select      parent.*
from      category_subcategories as parent
having      parent.lft = (
               select      max(parent.lft)
               from     (
                         SELECT        parent.category_name
                         ,       parent.lft
                         ,       parent.rgt
                         FROM        category_subcategories      AS node,
                                category_subcategories      AS parent
                         WHERE        node.lft      > parent.lft
                         AND        node.lft      < parent.rgt
                         AND        node.category_name = 'Sofa'
                         ORDER BY  parent.lft     -- Is this a waste of effort?
                    )  AS got_name_lft_and_rgt
              )     AS got_lft
;What is the purpose of having the inner sub-query, the one I called got_name_lft_and_rgt?
Also, in Oracle, an ORDER BY clause in a sub-query doesn;t guarantee that any super-queries will keep that order. Why do you have an ORDER BY clause in the sub-query, and not in the main query?