Handling hierarchy in oracle sql query

Question

I have two tables:
The first contains COMPANY_ID and CATEGORY_ID
The second contains CATEGORY_ID and PARENT_CATEGORY_ID
I need to return a list containing COMPANY_ID and CATEGORY_ID. The entries, however, must come not only from the first table, but also include all the descendants listed in the second table.
So for example (and please read the example carefully) - let’s say in the first table I have one row:
COMPANY_ID | CATEGORY_ID
1000 | 1
In the second table, I have three rows:
CATEGORY_ID | PARENT_CATEGORY_ID
1 | null
2 | 1
3 | 2
4 | null
I want to have the following rows as a result:
COMPANY_ID | CATEGORY_ID
1000 | 1
1000 | 2
1000 | 3
How can I do this in oracle 10 in a single SQL statement ?

Answer 1

Hi,
Welcome to the forum!
Whenver you have a question, post your sample data in a form that people can use.
CREATE TABLE and INSERT statements are great:
CREATE TABLE  first
(       company_id     NUMBER (4)
,     category_id     NUMBER (4)
INSERT INTO first (company_id, category_id) VALUES (1000,   1);CREATE TABLE AS is good, too:
CREATE TABLE  second
AS
SELECT     1 AS category_id, NULL AS parent_category_id     FROM dual     UNION ALL
SELECT     2,               1                          FROM dual     UNION ALL
SELECT     3,               2                          FROM dual     UNION ALL
SELECT     4,               NULL                    FROM dual;Without that, people can't test their ideas, and are often unsure about your data.
You should also say what version of Oracle you're using. That's especially important with CONNECT BY queries. Every version since Oracle 7 has had significant improvements in how to do CONNECT BY queries. The query below works in Oracle 10 (and up).
For hierarchies and trees, use CONNECT BY
When you have to join tables and use CONNECT BY, it's usually more efficient to do one in a sub-query, and the other in another query.
In the example below, we're doing the CONNECT BY in sub-query cbq, and the join in the main query.
WITH     cbq     AS
     SELECT     category_id
     ,     CONNECT_BY_ROOT category_id     AS root_category_id
     FROM     second
     START WITH     parent_category_id     IS NULL
           AND     category_id          IN ( SELECT  category_id          -- Maybe; see note below
                                         FROM    first
     CONNECT BY     parent_category_id     = PRIOR category_id
SELECT  first.company_id
,     cbq.category_id
FROM     first
JOIN     cbq     ON     first.category_id     = cbq.root_category_id
;The 2nd condition in the START WITH clause won't change the results any; any rows that are excluded by that condition would also be excluded by the join condition. There can be a big differenece in performance, however. Try the query both ways, and use the 2nd condition if it helps.
Edited by: Frank Kulash on Feb 3, 2010 3:46 PM