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From: sad <sad@bankir.ru>
Organization: IT
To: pgsql-sql@postgresql.org
Subject: Re: a wierd query
Date: Mon, 17 May 2004 11:12:49 +0400
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> I forgot one situation:
>
> if I run the query like so:
>       select distinct on (task_id, begindate) task_id, workhour_id,
> begindate as date from workhour UNION
>       select distinct on (task_id, enddate) task_id, workhour_id, enddate
> as date from workhour I get yet another value: 2961 rows.
>
> So I got 3 different result sets for 3 different ways to run the query.
> Even in this last case the UNION doesn't seem to only return unique value=
s,
> and I will still need the top-level select.

if we suppose this situation possible to program in SQL
it causes data-loss in query
(i mean unpredictable query result:
if you have two records (f=3D1,b=3D2),(f=3D1,b=3D3)
"distinct ON f" makes a value of b meaningless)

So you MUST NOT select that way

P.S.  This situation means: you have wrong data structure.=20