David Siegal wrote:
> I would like to create an aggregate function that returns a concatenation
> of grouped values. It would be particularly useful if I could pass an
> optional delimiter into the aggregate function.
I've managed to do this in two stages:
1. Collect the set of values into an array. This can be done using a custom aggregate function, array_accum, which
isdemonstrated within the PostgreSQL manual: http://www.postgresql.org/docs/7.4/interactive/xaggr.html
But here it is again:
CREATE AGGREGATE array_accum ( sfunc = array_append, basetype = anyelement, stype = anyarray,
initcond= '{}' );
It makes me wonder why this isn't a built-in aggregate???
2. Convert the array to a string. Using the built-in function array_to_string:
http://www.postgresql.org/docs/7.4/interactive/functions-array.html
Example:
SELECT team_number, array_to_string(array_accum(member_name), ', ') AS members FROM team GROUP BY
team_number;
You can also go full round-trip (delimited string -> set) using the
builtin function: string_to_array, and a custom pl/pgSQL function:
CREATE FUNCTION array_enum(anyarray) RETURNS SETOF anyelement AS ' DECLARE array_a ALIAS FOR $1; subscript_v
integer; BEGIN FOR subscript_v IN array_lower(array_a,1) .. array_upper(array_a,1) LOOP RETURN NEXT
array_a[subscript_v]; END LOOP; RETURN; END;
' LANGUAGE 'plpgsql'
STRICT IMMUTABLE;
Example:
SELECT * FROM array_enum(string_to_array('one,two,three',','));
--
Mark Gibson <gibsonm |AT| cromwell |DOT| co |DOT| uk>
Web Developer & Database Admin
Cromwell Tools Ltd.
Leicester, England.
