如何在Postgresql中产生自己的集合function

CREATE FUNCTION join_two_with_comma(text, text) RETURNS text
    LANGUAGE sql
    AS $_$
    SELECT CASE
        WHEN $2 IS NULL OR $2 = '' THEN $1
        WHEN $1 IS NULL OR $1 = '' THEN $2
        ELSE $1 || ', ' || $2
    END
$_$;

CREATE AGGREGATE join_with_comma(text) (
    SFUNC = join_two_with_comma,
    STYPE = text,
    INITCOND = ''
);