Question

I am trying to create an aggregate function that concatenates numbers by grouping them. How can I go about it? Let’s say I have a table like this below.

Table Numbers
123
145
187
105

I want the outcome to look like

105_123_145_187

I know how to use group_concat separator _ if I am working in MySQL.

How can I do it in PostgreSQL?

Answer 1

There is already such function:

SELECT string_agg(num::text,'_')
FROM Numbers;

Details here: string_agg.

Tell me, if you use postgresql 8.4 or earlier version. I will show you, how to implement this function as custom aggregate.

UPD Custom aggregate:

CREATE OR REPLACE FUNCTION public.concat_delimited (text, text, text)
RETURNS text AS
$body$
  SELECT $1 || (CASE WHEN $1 = '' THEN '' ELSE $3 END) || $2;
$body$
LANGUAGE 'sql'
IMMUTABLE
RETURNS NULL ON NULL INPUT;

CREATE AGGREGATE public.text_concat (text, text)
(
  SFUNC = public.concat_delimited,
  STYPE = text
);