Re: Controlling order of evaluation?

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От Andre Maasikas
Тема Re: Controlling order of evaluation?
Дата
Msg-id 415A44DB.7080409@abs.ee
обсуждение исходный текст
Ответ на Controlling order of evaluation?  (Jerry LeVan <jerry.levan@eku.edu>)
Список pgsql-general
Jerry LeVan wrote:

> I have hoped that
>
> select * from annual_report(2003)
> union
> select * from annual_profit_loss(2003)
>
> would print the last select last ;( but it inserts the last selection
> alphabetically into the rows of the annual_report depending on the label
> field... I suppose I could use a label "zzGrand Totals", but that just
> does not look right.

I guess that it's the UNION operator that needs to elimiate duplicate rows
by definition and therefore does sorting to do this. From your
description it seems that you could use UNION ALL instead which might
work ok for you.

> Is there any way I can force the last select to appear last?
>
AFAIK the only sure way is to use ORDER BY if you have
some suitable column or can make one up. Usually using expressions
one can do pretty complicated stuff but it seems that in case of
UNION only select list items are allowed in ORDER BY. But then again you
can do it as subselect and then ....

select a from (
  select a,b from ...
  union all
  select a,b from ...
) u
order by case when b='Last' then 'zzzz' else b end;

Andre

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