Обсуждение: In a view, how do I cause fields with possible NULLs to be treated as a blank string in a replace operation?

Hello,

So I have am playing with a view to test the feasibility of a
technique for storing some data.

It basically goes something like this:

CREATE VIEW formatted_table AS
       SELECT name,
          replace(some_template, '@', some_type) AS some_field
         FROM some_table;

some_template is something like 'foo@bar' or 'foobar' (note the
missing template character).

some_type is a single letter like 'a' or 'b', or it can be NULL.

The above view works fine for rows where some_type is a letter, and
some_field ends up as 'fooabar' or whatever.

However, when some_type is NULL, some_field ends up as NULL as well.
I understand that this is expected behaviour, but how do I cause the
view to treat a some_type of NULL as an empty string, so that
some_field simply ends up as 'foobar'?

Hope that was clear.

On 27 June 2011 17:53, Asfand Qazi (Sanger Institute)
<aq2.sanger@gmail.com> wrote:
> Hello,
>
> So I have am playing with a view to test the feasibility of a
> technique for storing some data.
>
> It basically goes something like this:
>
> CREATE VIEW formatted_table AS
>       SELECT name,
>              replace(some_template, '@', some_type) AS some_field
>         FROM some_table;
>
> some_template is something like 'foo@bar' or 'foobar' (note the
> missing template character).
>
> some_type is a single letter like 'a' or 'b', or it can be NULL.
>
> The above view works fine for rows where some_type is a letter, and
> some_field ends up as 'fooabar' or whatever.
>
> However, when some_type is NULL, some_field ends up as NULL as well.
> I understand that this is expected behaviour, but how do I cause the
> view to treat a some_type of NULL as an empty string, so that
> some_field simply ends up as 'foobar'?
>
> Hope that was clear.

Try coalesce: http://www.postgresql.org/docs/9.0/static/functions-conditional.html#AEN15541

So if foo is a null value, and you used COALESCE(foo, 'bar'), the
output would be 'bar', otherwise it would be whatever the value of foo
is.

--
Thom Brown
Twitter: @darkixion
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Registered Linux user: #516935

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On Mon, Jun 27, 2011 at 5:58 PM, Thom Brown <thom@linux.com> wrote:
>
> Try coalesce: http://www.postgresql.org/docs/9.0/static/functions-conditional.html#AEN15541
>
> So if foo is a null value, and you used COALESCE(foo, 'bar'), the
> output would be 'bar', otherwise it would be whatever the value of foo
> is.
>


On Mon, Jun 27, 2011 at 5:59 PM, Rick Genter <rick.genter@gmail.com> wrote:
> Either use '' as some_type, or use COALESCE(some_type, '').
>


Thank you both, that worked fine.

--
Regards,
     Asfand Qazi
     Team 87 HTGT
     Wellcome Trust Sanger Institute