|From:||Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>|
|To:||Michael Glaesemann <grzm(at)seespotcode(dot)net>|
|Cc:||Richard Huxton <dev(at)archonet(dot)com>, Walter Cruz <walter(dot)php(at)gmail(dot)com>, pgsql-sql(at)postgresql(dot)org|
|Subject:||Re: "for SELECT DISTINCT, ORDER BY expressions must appear in select list" - is that the standart or a limitation of postgresql?|
|Views:||Raw Message | Whole Thread | Download mbox | Resend email|
Michael Glaesemann <grzm(at)seespotcode(dot)net> writes:
> On Feb 15, 2007, at 22:35 , Richard Huxton wrote:
>> Walter Cruz wrote:
>>> The error: ERROR: for SELECT DISTINCT, ORDER BY expressions must
>>> appear in select list is due to a standart implementarion or a design
>>> decision of postgres?
>> I think ORDER BY is defined to take place after DISTINCT, which
>> means there is no meaningful "number" for it to order by. You could
>> arbitrarily choose the first number encountered, but I can't see
>> what sense it would make to order by them.
> I believe the reason is that DISTINCT depends on a sort to determine
> uniqueness (distinctness), so it's a implementation detail that if
> you're going to include an ORDER BY, you also need to include the
> same columns in the ORDER BY in the DISTINCT clause.
No, there's actually a definitional reason for it. Consider
SELECT DISTINCT x FROM tab ORDER BY y;
For any particular x-value in the table there might be many different y
values. Which one will you use to sort that x-value in the output?
Back in SQL92 they avoided this problem by specifying that ORDER BY
entries had to reference output columns. SQL99 has some messy verbiage
that I think comes out at the same place as our restriction:
A) If K(i) is not equivalent to a <value expression>
immediately contained in any <derived column> in the
<select list> SL of <query specification> QS contained
in QE, then:
I) T shall not be a grouped table.
II) QS shall not specify the <set quantifier> DISTINCT
or directly contain one or more <set function
regards, tom lane
|Next Message||Phillip Smith||2007-02-15 21:47:14||Re: Retrieving 'Credit' when 'C'|
|Previous Message||chrisj||2007-02-15 16:55:58||Re: can someone explain confusing array indexing nomenclature|