create aggregates to concatenate

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);

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.