Обсуждение: NULL values and string

Hi, List!

I'm using PG8.1.

Are there any abilities to represent NULL values as string?

I'm doing something like this:

create function func(int4) returns varchar as'
declare
    num_value alias for $1;
    string_value varchar;
begin
  
   string_value := \'input value = \' || num_value;
   return string_value;

end
'language 'plpgsql';


If I einvoke my function with NULL argument, it return NULL. But I want 'input value = NULL'.
Of course, I can check input value like this:

if(num_value isnull) then
   string_value := \'input value = NULL\';
else
   string_value := \'input_value = \' || num_value;
end if;

But it is not laconic...


Sergey Karin

Sergey Karin schrieb:
> Hi, List!
>
> I'm using PG8.1.
>
> Are there any abilities to represent NULL values as string?
>
> I'm doing something like this:
>
> create function func(int4) returns varchar as'
> declare
>     num_value alias for $1;
>     string_value varchar;
> begin
>
>    string_value := \'input value = \' || num_value;
>    return string_value;
>
> end
> 'language 'plpgsql';
>
>
> If I einvoke my function with NULL argument, it return NULL. But I want
> 'input value = NULL'.
> Of course, I can check input value like this:
>
> if(num_value isnull) then
>    string_value := \'input value = NULL\';
> else
>    string_value := \'input_value = \' || num_value;
> end if;
>

You can use COALESCE()


create function func(int4) returns text as $$
declare
     num_value alias for $1;
begin
     return 'input value = ' || COALESCE(num_value,'NULL');
end
$$ language 'plpgsql';


(Id rather use more descriptive name for your function)

Regards
Tino

Sergey Karin wrote:
> Hi, List!
>
> I'm using PG8.1.
>
> Are there any abilities to represent NULL values as string?

Null isn't a real value. Try not to think of it as a value.

http://archives.postgresql.org/pgsql-sql/2003-01/msg00222.php

>     num_value alias for $1;

>    string_value := \'input value = \' || num_value;

> If I einvoke my function with NULL argument, it return NULL. But I want
> 'input value = NULL'.

Because NULL means unknown. A string with an unknown string appended to
it is itself unknown.

> Of course, I can check input value like this:
>
> if(num_value isnull) then
>    string_value := \'input value = NULL\';
> else
>    string_value := \'input_value = \' || num_value;
> end if;
>
> But it is not laconic...

Try something like:
   string_value := ''input_value = '' || COALESCE(num_value, 'a null');

--
   Richard Huxton
   Archonet Ltd

Richard Huxton wrote:

> Sergey Karin wrote:
>
>> Are there any abilities to represent NULL values as string?
>
>
> Null isn't a real value. Try not to think of it as a value.
>
That being said, and with due credit elsewhere (http://www.varlena.com/varlena/GeneralBits/84.php), what I do is


CREATE OR REPLACE FUNCTION public.textcat_null(text, text)
  RETURNS text AS
'SELECT textcat(COALESCE($1, ''''), COALESCE($2, ''''));'
LANGUAGE sql' VOLATILE;



CREATE OPERATOR public.||+(
  PROCEDURE = "public.textcat_null",
  LEFTARG = text,
  RIGHTARG = text);

This goes against proper form, considering what NULL is designed for, but it sure is convenient.


Regards,
Berend Tober