Re: VIEW / ORDER BY + UNION

On Wed, Feb 23, 2005 at 19:33:07 +0100,
KÖPFERL Robert <robert(dot)koepferl(at)sonorys(dot)at> wrote:
>
> Otherwise you can treat this as a subselect and suround it with another
> select.
>
> Like
> select * from (<here goes your ex.>) order by orderno;

That is still a bad idea in this case. He will end up scanning the table
three times to pick up the three days and there will be a sort for each
union to remove duplicates (which there shouldn't be if orderno is a
candidate key).

>
> C:\> -----Original Message-----
> C:\> From: Bruno Wolff III [mailto:bruno(at)wolff(dot)to]
> C:\> Sent: Mittwoch, 23. Februar 2005 18:20
> C:\> To: WeiShang
> C:\> Cc: pgsql-sql(at)postgresql(dot)org
> C:\> Subject: Re: [SQL] VIEW / ORDER BY + UNION
> C:\>
> C:\>
> C:\> On Thu, Feb 17, 2005 at 23:46:59 +0800,
> C:\> WeiShang <thanks(at)verymuch(dot)com> wrote:
> C:\> > Hi, I have created a view like this :
> C:\> >
> C:\> > CREATE VIEW v1 AS (SELECT orderno,weekday,time FROM t1,t2 where
> C:\> > t1.orderno=t2.orderno);
> C:\> >
> C:\> > if I create a SQL statment:
> C:\> >
> C:\> > (SELECT orderno FROM v1 WHERE weekday='MON' ORDER BY orderno)
> C:\> > UNION
> C:\> > (SELECT orderno FROM v1 WHERE weekday='WED' ORDER BY orderno)
> C:\> > UNION
> C:\> > (SELECT orderno FROM v1 WHERE weekday='FRI' ORDER BY orderno);
> C:\> >
> C:\> > Will the whole result will be sorted by the field orderno?
> C:\>
> C:\> If this isn't a made up example, you don't want to do this. You
> C:\> should use IN or OR to select records corresponding to the days
> C:\> of interest and then use ORDER BY to select the ordering.
> C:\>
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