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From "Alex Miller" <amil...@metamatrix.com>
Subject RE: [jira] Commented: (DERBY-883) Enhance GROUP BY clause to support expressions instead of just column references.
Date Thu, 18 May 2006 13:57:41 GMT
I blogged about GROUP BY expression support across dbs a while back
(http://tech.puredanger.com/2005/03/02/fun-with-expressions-in-a-group-b
y/ and
http://tech.puredanger.com/2005/04/01/update-on-expressions-in-group-by/
).  Pretty much all the major vendors support expressions in a GROUP BY.
I implemented this functionality a while back for the MetaMatrix query
engine and blogged about using MetaMatrix Query to "add" this
functionality over Derby in
http://devcentral.metamatrix.com/blog/alex/2006/02/28/Enhancing-Apache-D
erby-with-MetaMatrix.

Positional parameters don't make as much sense for GROUP BY as they do
for ORDER BY.  Logically, you're executing the clauses in the order
FROM-WHERE-GROUP BY-HAVING-SELECT-ORDER BY.  The SELECT clause is the
point at which the output columns of the query are effectively named and
ordered.  Positional parameters make sense in ORDER BY because they
refer to the output columns of the previous phase, which are defined as
part of the query, but may have no well-defined name to refer to them
with.  

In the case of GROUP BY, you would be referring backwards from the
SELECT clause to the GROUP BY clause, so that seems kind of goofy.  The
only reason to do this would be to avoid referencing a complex unnamed
expression.  This is, of course, exactly the sort of thing that db
vendors bend the rules about to make SQL more usable.  In fact, you can
use GROUP BY positional parameters in MySQL and Postgres but not in any
major commercial db that I've tried (Oracle, DB2, SQL Server, Sybase).  

Alex Miller
Chief Architect
MetaMatrix

-----Original Message-----
From: Satheesh Bandaram (JIRA) [mailto:derby-dev@db.apache.org] 
Sent: Wednesday, May 17, 2006 1:14 PM
To: derby-dev@db.apache.org
Subject: [jira] Commented: (DERBY-883) Enhance GROUP BY clause to
support expressions instead of just column references.

    [
http://issues.apache.org/jira/browse/DERBY-883?page=comments#action_1241
2227 ] 

Satheesh Bandaram commented on DERBY-883:
-----------------------------------------

Thanks for making progress on this important new functionality, Manish.

> 1. In terms of syntax, do we allow expressions in the group by list or
positional parameters, or both?
>
> select tomonth(creationdt), toyear(creationdt), count(*) from bugs 
> group by 1, 2;

I have seen positional parameters for ORDER BY expressions, not
typically used in GROUP BY. Looking at both DB2 and Oracle
documentation, it seems neither support positional parameters.

> An implementation question on this note-- does the language code have 
> a way of looking at two expressions (ValueNode?) and checking to see 
> if they are equivalent? We'll need some way of doing this to match an 
> expression in the group by list to an expression in the select list
right?

Correct. Don't think there is any existing expression matching to
compare two expressions. DB2 docs discuss how group by expressions are
matched in SQL reference manual. (Page 484:
ftp://ftp.software.ibm.com/ps/products/db2/info/vr82/pdf/en_US/db2s1e81.
pdf)

> 2. I assume that an expression in a group by list must appear in the 
> select list without aggregation right? Is this an error?
> 
> select x+1, x+2, sum(y)
> from test
> group by x

NO... This is a valid query. See the reference I provided above. 

> 3. What do we do with duplicates? i.e.
> 
> select x+1, x+1, sum(y)
> from test
> group by x+1, x+1;
> 
> Is this an error? The current implementation throws an error if the 
> same column occurs more than once in the group by list.

I am not sure why Derby currently considers this an error... Looking at
the code, it seems it may be looking for ambiguous column references
(like 'x' being part of two different tables in from_list), which makes
sense, but not sure why duplicate references should be prevented.

> Is there a standard somewhere which I should consult before trying to
nail down the functionality? 

Unfortunately, NO.... SQL 2003 seems to allow only column references in
GROUP BY clause. But both DB2 and Oracle allow expressions in GROUP BY
list and likely allowed by other database vendors too. You could use
either DB2 or Oracle docs to understand how this functionality is
defined there. Much easier to read these docs than confusing SQL 2003
spec.


> Enhance GROUP BY clause to support expressions instead of just column
references.
> ----------------------------------------------------------------------
> -----------
>
>          Key: DERBY-883
>          URL: http://issues.apache.org/jira/browse/DERBY-883
>      Project: Derby
>         Type: New Feature

>   Components: SQL
>     Versions: 10.1.2.1
>  Environment: JDK 1.5.0_05
>     Reporter: Lluis Turro
>     Assignee: Manish Khettry

>
> This select would return an error syntax on finding "(" after month if
group by clause:
> select idissue, month(creation), year(creation), count(distinct 
> idissue) where
>   ....
> group by idissue, month(creation), year(creation)

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