[jira] [Commented] (DERBY-11) Recursive SQL and WITH A(col list) as (Select col list From Table List) Support.

["Rick Hillegas (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DERBY-11?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13861621#comment-13861621 ] 

Rick Hillegas commented on DERBY-11:
------------------------------------

Some overviews of recursive queries can be found here:

http://en.wikipedia.org/wiki/Hierarchical_and_recursive_queries_in_SQL 

http://walkingoncoals.blogspot.com/2009/12/fun-with-recursive-sql-part-1.html

http://gennick.com/database/understanding-the-with-clause

The 2011 SQL Standard provides language for recursive queries as follows:

Recursive queries are defined by

T121) WITH (excluding RECURSIVE ) in query expression

T122) WITH (excluding RECURSIVE) in subquery

T131) Recursive query

T132) Recursive query in subquery


Relevant sections of part 2 include

1) The withClause as defined in section 7.13 (query expression).

2) The searchOrCycleClause as defined in section 7.14

I believe that both Oracle (as of 11g) and DB2 (as of some version) support the SQL Standard syntax. Or at least claim to support it.

The following looks to me like it might be a legal, example SQL Standard recursive query:

{noformat}
WITH RECURSIVE emp_hier (emp_id, mgr_id, level) AS
(
SELECT a.emp_id, a.mgr_id, 0 
FROM   employee a
WHERE  a.emp_id = 123
UNION ALL
SELECT b.emp_id, b.mgr_id, c.level+1 
FROM   employee b,
       emp_hier c
WHERE  b.mgr_id = c.emp_id
)
SELECT e.emp_title, e.emp_id, e.mgr_id, h.level
FROM   employee e,
       emp_hier h
WHERE  e.emp_id = h.emp_id
  AND  e.emp_id <> 123;
{noformat}

My gut feeling is that this feature would not entail more work than has been involved in implementing deferrable constraints (DERBY-532) or the MERGE statement (DERBY-3155).


> Recursive SQL and WITH A(col list) as (Select col list From Table List) Support.
> --------------------------------------------------------------------------------
>
>                 Key: DERBY-11
>                 URL: https://issues.apache.org/jira/browse/DERBY-11
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>         Environment: ANY.
>            Reporter: Ali Demir
>              Labels: derby_triage10_11
>
> Right now, in Derby, there is no way to define a temporary Result Set to use in subsequent statements. This makes complicated concepts to be expressed in SQL either very very complicated and lengthy or simply impossible.
> DB2 has a simple and useful syntax using a "WITH" statement. It would be nice if Derby can support this. An example is as below:
> WITH A(COL1, COL2) as (SELECT COL1, COL2 FROM T1 WHERE condition)
> SELECT T2.COL3 FROM T2, A WHERE condition2
> It can be extended to include more WITH clauses:
> WITH A(COL1, COL2) as (SELECT COL1, COL2 FROM T1 WHERE condition)
> WITH B(COL3) as (SELECT COL3 FROM T1,A WHERE condition2)
> SELECT T2.COL5, B.COL3 FROM T2, A, B WHERE condition3
> and so on.
> Note that as the following example shows, the use of table correlation name in another subselect is NOT supported and cannot be a workaround:
> SELECT cols FROM (SELECT cols FROM T1) as A, (SELECT cols FROM T2,A where A relates to T2) as B where condition
> Another interesting aspect of these WITH clauses is their ability to make RECURSIVE SQL possible. In below example, definition of A includes a select from ITSELF:
> WITH A(COL1, COL2) as (SELECT COL1, COL2 FROM T1 UNION ALL SELECT COL1, COL2 FROM T2, A where A.COL1=T2.COLN)
> SELECT COL1, COL2 FROM A WHERE condition2
> Recursion with a WITH clause relies on a specific syntax. Consult DB2 documentation for more info about Recursion and WITH clause. 
> Recursion is an important facility and it would be very very useful to have it in Derby.
> Recursion comes in very handy when a single table holds a hierarchy of rows that are related to each other with parent-child relationships of N-Levels where N is large or unknown in which case non-recursive solutions are either impossible or require complicated code at the Client side. With recursion possible at the SQL level, many problems can be reduced to single SQL statements instead of lengthy application code.
> Regards,
> Suavi Demir



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