Re: create aggregates to concatenate

["Berend Tober" <btober@xxxxxxxxxxxxxxxx>]

>> i just wanted to share this with you, i wanted to do something like
>> this for a long time but just recently found out about "create
>> aggregate" reading old posts, so here it is, using user-defined
>> aggregate functions to concatenate results.
>>
>> when it's numbers i usually use SUM to compute totals, but when it's
>> text you can create your own aggregate function to concatenate:
>>
>> CREATE FUNCTION concat (text, text) RETURNS text AS $$
>>   DECLARE
>>     t text;
>>   BEGIN
>>     IF character_length($1) > 0 THEN
>>       t = $1 ||', '|| $2;
>>     ELSE
>>       t = $2;
>>     END IF;
>>     RETURN t;
>>   END;
>> $$ LANGUAGE plpgsql;
>>
>> CREATE AGGREGATE pegar (
>>   sfunc = concat,
>>   basetype = text,
>>   stype = text,
>>   initcond = ''
>> );
>>
>> then, for instance to list the countries names followed by the cities
>> in those countries as a comma separated list, you can use something
>> like (assuming you have those tables and "pais" is a foreign key in...
>> etc):
>>
>> SELECT paises.pais, pegar(ciudad) FROM ciudades JOIN paises ON
>> ciudades.pais=paises.pais GROUP BY paises.pais
>>
>> if i'm missing something or doing something wrong please let me know,
>> this is my first aggregate function.
>>
>
> And, while somewhat off-topic but in a similar vein, although the following
> goes against the SQL standard so dearly held to by the Postgresql team, I
> found it useful in some cirumstances to circumvent the handling of NULL's in
> text columns with
>
> 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);
>

Slightly less off-topic:

-- Try this

CREATE TABLE country (country_name varchar(64) NOT NULL);

INSERT INTO country VALUES ('Afghanistan');
INSERT INTO country VALUES ('Albania');
INSERT INTO country VALUES ('Algeria');
INSERT INTO country VALUES ('Andorra');
INSERT INTO country VALUES ('Angola');
INSERT INTO country VALUES ('Anguilla');
INSERT INTO country VALUES ('Argentina');
INSERT INTO country VALUES ('Armenia');
INSERT INTO country VALUES ('Aruba');
INSERT INTO country VALUES ('Ascension');
INSERT INTO country VALUES ('Australia');
INSERT INTO country VALUES ('Austria');

-- ... etc., etc.

CREATE AGGREGATE concat (
    BASETYPE = text,
    SFUNC = textcat,
    STYPE = text,
    INITCOND = ''
);


SELECT TRIM(', ' FROM (SELECT CONCAT(country_name||', ') FROM COUNTRY));

-- to get a comma-separated list of country names.



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