From Mark.Fenbers@noaa.gov Mon May  9 16:58:04 2005
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Date: Mon, 09 May 2005 12:57:41 -0400
From: "Mark Fenbers" <Mark.Fenbers@noaa.gov>
Organization: DoC/NOAA/NWS/OHRFC
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Subject: default value for select?
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I want to update a column in myTable.&nbsp; The value this column is set to
depends on a nested select statement which sometimes returns 0 rows
instead of 1.&nbsp; This is a problem since the column I'm trying to update
is set to refuse nulls.&nbsp; Here's a sample:<br>
<br>
update myTable set myColumn = (Select altColumn from altTable where
altColumn != 'XXX' limit 1) where myColumn = 'XXX';<br>
<br>
MyColumn cannot accept nulls, but sometimes "Select altColumn ..."
returns 0 rows, and thus, the query fails.&nbsp; <br>
<br>
Is there a way to set a default value to be inserted into myColumn if
and when "select altColumn ..." returns zero rows?<br>
<br>
Mark<br>
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From postgresql@philip.pjkh.com Mon May  9 17:35:14 2005
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From: Philip Hallstrom <postgresql@philip.pjkh.com>
To: Mark Fenbers <Mark.Fenbers@noaa.gov>
Cc: pgsql-sql@postgresql.org
Subject: Re: default value for select?
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> I want to update a column in myTable.  The value this column is set to depends on a
> nested select statement which sometimes returns 0 rows instead of 1.  This is a
> problem since the column I'm trying to update is set to refuse nulls.  Here's a
> sample:
> 
> update myTable set myColumn = (Select altColumn from altTable where altColumn !=
> 'XXX' limit 1) where myColumn = 'XXX';
> 
> MyColumn cannot accept nulls, but sometimes "Select altColumn ..." returns 0 rows,
> and thus, the query fails. 
> 
> Is there a way to set a default value to be inserted into myColumn if and when
> "select altColumn ..." returns zero rows?

COALESCE(value [, ...])

The COALESCE function returns the first of its arguments that is not null. 
Null is returned only if all arguments are null. This is often useful to 
substitute a default value for null values when data is retrieved for 
display, for example:

SELECT COALESCE(description, short_description, '(none)') ...

Like a CASE expression, COALESCE will not evaluate arguments that are not 
needed to determine the result; that is, arguments to the right of the 
first non-null argument are not evaluated.



From keithw@narrowpathinc.com Mon May  9 17:27:31 2005
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From: "Keith Worthington" <keithw@narrowpathinc.com>
To: "Mark Fenbers" <Mark.Fenbers@noaa.gov>
Cc: pgsql-sql@postgresql.org
Reply-To: KeithW@narrowpathinc.com
Subject: Re: default value for select?
Date: Mon, 9 May 2005 13:26:55 -0400
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On Mon, 09 May 2005 12:57:41 -0400, Mark Fenbers wrote
> I want to update a column in myTable. The value this column is set 
> todepends on a nested select statement which sometimes returns 0 
> rowsinstead of 1. This is a problem since the column I'm trying to 
> updateis set to refuse nulls. Here's a sample:
> 
> update myTable set myColumn = (Select altColumn from altTable 
> wherealtColumn != 'XXX' limit 1) where myColumn = 'XXX';
> 
> MyColumn cannot accept nulls, but sometimes "Select altColumn 
> ..."returns 0 rows, and thus, the query fails.
> 
> Is there a way to set a default value to be inserted into myColumn 
> ifand when "select altColumn ..." returns zero rows?
> 
> Mark

Mark,

I do not know if it will work but I would try the COALESCE function.
http://www.postgresql.org/docs/8.0/interactive/functions-conditional.html

Kind Regards,
Keith



From ajwasson@gmail.com Mon May  9 17:30:58 2005
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From: Tony Wasson <ajwasson@gmail.com>
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To: Mark Fenbers <Mark.Fenbers@noaa.gov>
Subject: Re: default value for select?
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On 5/9/05, Mark Fenbers <Mark.Fenbers@noaa.gov> wrote:
>  I want to update a column in myTable.  The value this column is set to
> depends on a nested select statement which sometimes returns 0 rows inste=
ad
> of 1.  This is a problem since the column I'm trying to update is set to
> refuse nulls.  Here's a sample:
> =20
>  update myTable set myColumn =3D (Select altColumn from altTable where
> altColumn !=3D 'XXX' limit 1) where myColumn =3D 'XXX';
> =20
>  MyColumn cannot accept nulls, but sometimes "Select altColumn ..." retur=
ns
> 0 rows, and thus, the query fails. =20
> =20
>  Is there a way to set a default value to be inserted into myColumn if an=
d
> when "select altColumn ..." returns zero rows?
> =20
>  Mark

Mark,=20
You can work around this by using a CASE statement. In this case, test
for a NULL from your subquery. This is not elegant at all, but it
should do what you are wanting.

update myTable set myColumn =3D (CASE
  WHEN (Select altColumn from altTable where  altColumn !=3D 'XXX'
limit 1) IS NULL
    THEN 'some default value'
  ELSE (Select altColumn from altTable where  altColumn !=3D 'XXX' limit 1)
  END)
where myColumn =3D 'XXX';

Hope this helps...
Tony