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Thread: UNION and ORDER BY ... IS NULL ASC

UNION and ORDER BY ... IS NULL ASC

From
"A.M."
Date:
  PostgreSQL 7.3.2 on i686-pc-linux-gnu, compiled by GCC 2.95.3

I have come across some unexpected behavior while dealing with a UNION 
and ORDER BY. I'd like some advice. Here's a scenario where I want to 
order by null values:

CREATE TABLE test(a int);
SELECT a FROM test UNION SELECT a FROM test ORDER BY a IS NULL ASC;

returns:

ERROR:  ORDER BY on a UNION/INTERSECT/EXCEPT result must be on one of 
the result columns

whereas:

SELECT a FROM test UNION SELECT a FROM test ORDER BY a;

works fine. The column name is the same in both queries, yet I get an 
error! Obviously, this is a gross oversimplification of what I want to 
do, but I couldn't get it working in this minimal case. I also tried 
using the column number, and that returns the same results as the name. 
What am I doing wrong? Thanks for any info.><><><><><><><><><
AgentM
agentm@cmu.edu

Re: UNION and ORDER BY ... IS NULL ASC

From
Stephan Szabo
Date:
On Fri, 4 Apr 2003, A.M. wrote:

>   PostgreSQL 7.3.2 on i686-pc-linux-gnu, compiled by GCC 2.95.3
>
> I have come across some unexpected behavior while dealing with a UNION
> and ORDER BY. I'd like some advice. Here's a scenario where I want to
> order by null values:
>
> CREATE TABLE test(a int);
> SELECT a FROM test UNION SELECT a FROM test ORDER BY a IS NULL ASC;
>
> returns:
>
> ERROR:  ORDER BY on a UNION/INTERSECT/EXCEPT result must be on one of
> the result columns
>
> whereas:
>
> SELECT a FROM test UNION SELECT a FROM test ORDER BY a;
>
> works fine. The column name is the same in both queries, yet I get an
> error! Obviously, this is a gross oversimplification of what I want to
> do, but I couldn't get it working in this minimal case. I also tried
> using the column number, and that returns the same results as the name.
> What am I doing wrong? Thanks for any info.

Try something like:

select a from (select a from test union select a from test) as foo order
by a is null asc;

It's an unsupported extension (I don't think SQL would allow
"order by a is null" in the first place) so you'll have to workaround for
now.

Re: UNION and ORDER BY ... IS NULL ASC

From
Josh Berkus
Date:
AM,

> CREATE TABLE test(a int);
> SELECT a FROM test UNION SELECT a FROM test ORDER BY a IS NULL ASC;
>
> returns:
>
> ERROR:  ORDER BY on a UNION/INTERSECT/EXCEPT result must be on one of
> the result columns

I do believe that this is per the SQL spec; UNIONs may only be sorted on the
output columns.

So if you did
SELECT a, (a IS NULL) as test_a
FROM test
UNION
SELECT a, (a IS NULL)
FROM test2
ORDER BY test_a

... it should work.

--
-Josh BerkusAglio Database SolutionsSan Francisco

Re: UNION and ORDER BY ... IS NULL ASC

From
Tom Lane
Date:
Josh Berkus <josh@agliodbs.com> writes:
>> ERROR:  ORDER BY on a UNION/INTERSECT/EXCEPT result must be on one of 
>> the result columns

> I do believe that this is per the SQL spec; UNIONs may only be sorted on the 
> output columns.

Right.  We extend the SQL spec for ORDER BY on simple selects, but we
have not gotten around to doing so for ORDER BY on set-operation
results.  You can only sort by unadorned output columns.

Patches to improve this are welcome ;-)
        regards, tom lane