From: Andreas Joseph Krogh <andreak@officenet.no>
Organization: OfficeNet AS
To: pgsql-sql@postgresql.org
Subject: Problem with UNION-queries
Date: Mon, 12 Nov 2007 21:27:14 +0100
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Hi all

I have a somewhat furry solution to a problem for which I think there might=
 be=20
a better way to do it.

The table looks like this (simplified for the sake of this example):

drop table if exists test1;
create table test1(
id serial primary key,
key varchar,
username varchar,
value varchar
);

=2D- Insert test-data
insert into test1(username, key, value) values('andreak', 'A', 'value1');
insert into test1(username, key, value) values('andreak', 'A', 'value2');
insert into test1(username, key, value) values('andreak', 'A', 'value3');
insert into test1(username, key, value) values('andreak', 'B', 'value2');
insert into test1(username, key, value) values('andreak', 'B', 'value3');
insert into test1(username, key, value) values('andreak', 'B', 'value4');
insert into test1(username, key, value) values('andreak', null, 'value3');
insert into test1(username, key, value) values('andreak', null, 'value4');
insert into test1(username, key, value) values('andreak', null, 'value5');

=46or the sake of this example the test-case is greatly simplified, so I ha=
ve=20
the following query to give me all rows for value=3D'A' and value=3D'B'

select t.username, t.key, t.value from test1 t where t.key =3D 'A'
UNION
select t.username, t.key, t.value from test1 t where t.key =3D 'B'

 username | key | value
=2D---------+-----+--------
 andreak  | A   | value1
 andreak  | A   | value2
 andreak  | A   | value3
 andreak  | B   | value2
 andreak  | B   | value3
 andreak  | B   | value4
(6 rows)

Again, I know there are other, better, ways to accomplish that with this=20
simple schema, but again it's for the sake of the example. The important=20
thing here is that it's 2 UNION-queries providing the result.

Now - what I'm trying to accomplish is getting the following result-set:
 username | key | value
=2D---------+-----+--------
 andreak  | A   | value1
 andreak  | A   | value2
 andreak  | A   | value3
 andreak  | B   | value2
 andreak  | B   | value3
 andreak  | B   | value4
 andreak  |     | value5
(7 rows)

That is - I want all rows with username=3D'andreak' AND (key IS NULL) where=
=20
the "value" is not in the previous result (not part of the other=20
UNION-queries). The hard, and important, part is that the resulting=20
rows' "value" must not exist in the "value"-column for any previous rows.

Here is one way I figured out how to do it:

select t.username, t.key, t.value from test1 t where t.key =3D 'A'
UNION
select t.username, t.key, t.value from test1 t where t.key =3D 'B'
UNION
select t.username, t.key, t.value from test1 t where t.value NOT IN (
  select value from (
    select t.username, t.key, t.value from test1 t where t.key =3D 'A'
    UNION
    select t.username, t.key, t.value from test1 t where t.key =3D 'B'
  ) tmp1
)
;

Given that my real schema is way more complex I'm looking for a solution wh=
ich=20
doesn't involve issuing the NOT IN (original UNION-query) as it is a rather=
=20
heavy query.

Does anybody have a better approach to this problem?

=2D-=20
Andreas Joseph Krogh <andreak@officenet.no>
Senior Software Developer / Manager
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