Re: [SQL] Trouble with null text fields

Glenn Waldron ha scritto:

> Using Postgres 6.5 beta (snap 12 apr), on Linux i386. I moved up from
> 6.4.2 when I couldn't get things working.
>
> I'm having difficulty dealing with null text/varchar fields. I need
> to be able to interpret null values as the null string '' for the
> purposes on concatenation.
>
> 1) ----
> For example, the query:
>
> SELECT (field_one || field_two) from t1;
>
> Will return the concatenation of the two fields. If either of the fields
> is null, it is interpreted as the empty string '' and the correct answer
> is printed. But:
>
> SELECT * from t1 where ( field_one || field_two = 'something' )
>
> This does NOT work is either field_one or field_two is null. Same result
> with the textcat() function.
>
> 2) ----
> Next I tried using "case", getting a parse error at or near "then":
>
> SELECT ( case field_one when null then '' else field_one end ) from t1;
>

This is the syntax that PostgreSQL understands:

select case when field_one is null then t||'?' else t||v end from t1;

>
> This one gave me "ERROR: copyObject: don't know how to copy 704":
>
> SELECT ( case field_one when 'string' then 'other' else 'third' end)
> from t1;
>

Seems that PostgreSQL doesn't recognize this syntax yet.

>
> 3) ---
> I tried writing a function that takes a "text" type and returns '' is the
> string
> is null. Never could successfully do a null test on a function parameter.
>
> 4) ----
> I also tried writing my own concat function, and found that passing
> null fields into a user function doesn't seem to work either. So I tried
> passing the whole thing in as a TUPLE, since you can determine whether a
> field is null with the GetAttributeByName() call.
>
> The creation:
>
> CREATE FUNCTION mycat(text,text) returns text as '/usr/.../file.so'
> langauge 'sql';
>
> This worked great, even with null values:
>
> SELECT mycat(field_one, field_two) from t1;
>
> This crashed the backend, with a "Memory exhauted in AllocSetAlloc()" error:
>
> SELECT * from t1 where mycat(field_one, field_two) = 'something';
>
> So I tried making an index, and got: "DefineIndex(): Attibute t1 not found"
>
> CREATE INDEX t1_ix on t1 ( mycat(t1) text_ops );
>
> Any help is appreciated!! Sorry for the novel!! -glenn

Try this one:

create function coalesce(text) returns text as
'declare
nonullo ALIAS FOR $1;
begin
IF nonullo ISNULL THEN
RETURN ''\\\\N'';
ELSE
RETURN nonullo;
END IF;
end;
' language 'plpgsql';