From: Aaron Bono <postgresql@aranya.com>
To: Postgres SQL language list <pgsql-sql@postgresql.org>
Subject: Unions and Grouping
Date: Fri, 15 Dec 2006 15:15:12 -0600
Message-ID: <bf05e51c0612151315s59e6647etc2dd45e940d41a2a@mail.gmail.com> (raw)
I have a question about the SQL Specifications in regards to Unions...
I recently put together a query that involved unions similar to the
following:
SELECT
'Query 1' as id,
my_value
FROM my_view
UNION
SELECT
'Query 2' as id,
my_value
FROM my_other_view
;
The first query in the union gave me 39 records and the second gave me 34
records. I was expecting the union to give me 39 + 34 = 73 records.
When I ran this against DB2, I got 35 records (not sure about PostgreSQL -
will have to try it when I get home). What I found was when I did a group
by my_value on each query I got two values that then added to 35. The
reason was, my_value was duplicated in my_view and in my_other_view. What
the Union appeared to be doing was to gather the data and then do a group by
on the complete results. I expected it to only eliminate duplicates BETWEEN
the two queries, not WITHIN the queries.
My question, what do the SQL Specifications say should happen on a Union?
Is it supposed to eliminate duplicates even WITHIN the individual queries
that are being unioned?
Thanks!
--
==================================================================
Aaron Bono
Aranya Software Technologies, Inc.
http://www.aranya.comhttp://codeelixir.com
==================================================================
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To: pgsql-sql@postgresql.org
Cc: postgresql@aranya.com
Subject: Re: Unions and Grouping
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