Re: subquery join order by

On 19 November 2010 01:36, Mage <mage(at)mage(dot)hu> wrote:
>            Hello,
>
> (I googled and read docs before sending this e-mail).
>
> Is it necessary to use order by twice (inside and outside) to get the proper
> order if I have an ordered subqery in a join?
>
> select * from (select distinct on (b_id) * from a order by b_id, id) sub
> left join b on b.id = sub.b_id;
>
> or
>
> select * from (select distinct on (b_id) * from a order by b_id, id) sub
> left join b on b.id = sub.b_id order by b_id;
>
>
> It seems to me that it's enough to use 'order by' only inside wheter 'by
> desc' or 'by asc' (b_id), however I'd like to be sure.
>
> Thank you.
>
>            Mage

You should always use ORDER BY on the outer-most part of the query
since that's what will be finally returning your data. Don't bother
with ordering sub-selects.

So in your case, just use:

SELECT *
FROM (SELECT DISTINCT ON (b_id) * FROM a) sub
LEFT JOIN b ON b.id = sub.b_id
ORDER BY sub.b_id, sub.id;

But why bother with a sub-select anyway? You can write it as:

SELECT DISTINCT ON (a.b_id) *
FROM a
LEFT JOIN b ON b.id = a.b_id
ORDER BY a.b_id, a.id;

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Thom Brown
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