Thread: default value for select?

I want to update a column in myTable.  The value this column is set to depends on a nested select statement which
sometimesreturns 0 rows instead of 1.  This is a problem since the column I'm trying to update is set to refuse nulls. 
Here'sa sample:<br /><br /> update myTable set myColumn = (Select altColumn from altTable where altColumn != 'XXX'
limit1) where myColumn = 'XXX';<br /><br /> MyColumn cannot accept nulls, but sometimes "Select altColumn ..." returns
0rows, and thus, the query fails.  <br /><br /> Is there a way to set a default value to be inserted into myColumn if
andwhen "select altColumn ..." returns zero rows?<br /><br /> Mark<br /> 

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

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 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?
>
>  Mark

Mark,
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 = (CASE WHEN (Select altColumn from altTable where  altColumn != 'XXX'
limit 1) IS NULL   THEN 'some default value' ELSE (Select altColumn from altTable where  altColumn != 'XXX' limit 1)
END)
where myColumn = 'XXX';

Hope this helps...
Tony

> 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.