On 12/15/06,
Ragnar <
gnari@hive.is> wrote:
On fös, 2006-12-15 at 16:17 -0600, Aaron Bono wrote:
> On 12/15/06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Aaron Bono" < postgresql@aranya.com> writes:
> > 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?
>
> Yes. SQL92 7.10 saith:
> [snip SQL92 standardese]
>
> So if I don't want the duplicated WITHIN the two queries to be
> eliminated, I use UNION ALL?
with UNION ALL, no duplicates will be discarded, either
"within" or "between" the queries.
if I understand you, you are looking for having only queries
that are both in Q1 and Q2 removed, but not for example,
any duplicates in Q1 that are not in Q2.
if this is what you want, then I think you may not have
thought it through. what about a row that is twice in
Q1 and three times in Q2? how many copies of this row
should be in the result?
gnari
What I want is two-fold:
1. I want to eliminate all duplicates which is exactly what UNION does - so I am using a straight UNION
2. I want to understand how UNION and UNION ALL work (which I do now - thanks)
I didn't really want the duplicates between the two queries eliminated but not within. That is just what I had expected to see the first time I ran the query. As you pointed out, this really isn't desirable so I am glad it doesn't work this way.
Thanks for all the input.
--
==================================================================
Aaron Bono
Aranya Software Technologies, Inc.
http://www.aranya.com http://codeelixir.com==================================================================