Richard, Andreas, thank you very much for your solutions. I took a look on both solutions, but I choosed Andreas's solution because is shorter :)
So Andreas, would you please give some more explanations on your solution? I didn't work with functions and aggregate till now. I don't understand how this comma_aggregate works. I can see that this function it is defined with two arguments, but the aggredate it is called with a single argument. And what is LANGUAGE sql IMMUTABLE STRICT ? CREATE FUNCTION comma_aggregate(text,text) RETURNS text AS '
SELECT CASE WHEN $1 <> '''' THEN $1 || '', '' || $2 ELSE $2 END; ' LANGUAGE sql IMMUTABLE STRICT; CREATE AGGREGATE comma (basetype=text, sfunc=comma_aggregate, stype=text, initcond='' );
Best regards, Loredana