Message-ID: <423A144B.2010304@ca.afilias.info>
Date: Thu, 17 Mar 2005 18:35:39 -0500
From: Andrew Hammond <ahammond@ca.afilias.info>
User-Agent: Mozilla Thunderbird 1.0 (Windows/20041206)
MIME-Version: 1.0
To: Leon Stringer <leon.stringer@ntlworld.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Consecutive row count query
References: <1111090884.22783.5.camel@localhost.localdomain>
In-Reply-To: <1111090884.22783.5.camel@localhost.localdomain>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

You could hack it using a custom aggregate. NB: you'll want to reset the 
  categorizer_seq every now and then. And this isn't safe for concurrent 
queries. You could make it safe for concurrent queries by using a 
complex type for STYPE, but I didn't bother. I also haven't debugged 
this, but I think it expresses the concept.


CREATE SEQUENCE categorizer_seq;

CREATE OR REPLACE FUNCTION categorizer_func (string, string) RETURNS 
bigint VOLATILE CALLED ON NULL INPUT AS '
SELECT CASE WHEN $1 = $2
     THEN (SELECT last_value FROM categorizer_seq)
     ELSE nextval(''categorizer_seq'')
     END AS category
' LANGUAGE SQL;

CREATE AGGREGATE categorizer (
     BASETYPE = text,
     SFUNC = categorizer_func,
     STYPE = text,
     INITCOND = ''
);

SELECT col1, count(*)
FROM (
     SELECT col1, cagetorizer(col1) AS category
     FROM mytable
     ORDER BY col_order
) tmp
GROUP BY (col1, category);

Leon Stringer wrote:
> Hi,
> 
> I wondered if anyone could answer the following question:
> 
> If I have a table such as the one below:
> 
> col1   col_order
> -----------
> Apple  1
> Apple  2
> Orange 3
> Banana 4
> Apple  5
> 
> Is there a way I can get the following results:
> 
> Apple  2
> Orange 1
> Banana 1
> Apple  1
> 
> i.e. Each row is printed ordered by col_order but consecutive
> appearances of the same col1 result in only a single line in the result
> with the number of consecutive appearances.
> 
> Obviously I could store the table as:
> 
> col1   col_order col_count
> --------------------------
> Apple  1         2
> Orange 2         1
> Banana 3         1
> Apple  4         1
> 
> But since (in my intended table) most rows will have col_count = 1, this
> seems like unnecessary normalization (and semantically "wrong").
> 
> Thanks in advance for any help,
> 
> Leon Stringer
> 
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 3: if posting/reading through Usenet, please send an appropriate
>       subscribe-nomail command to majordomo@postgresql.org so that your
>       message can get through to the mailing list cleanly