trigger function to replace insert with update

[whsdu]

This is a really simple question and I'v been working on it for 2 hours........

The table is:

CREATE TABLE download
(
  id integer NOT NULL DEFAULT nextval('seq'::regclass),
  songid integer NOT NULL,
  counter integer NOT NULL,
  weeknumber integer NOT NULL,
  CONSTRAINT download_pkey PRIMARY KEY (id ),
  CONSTRAINT download_songid_fkey FOREIGN KEY (songid)
      REFERENCES song (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
)
WITH (
  OIDS=FALSE
);
ALTER TABLE download
  OWNER TO postgres;

and what I want to do is whenever I want to insert a new record ,if the combination of songid and weeknumber is exist the counter add one otherwise execute the insert.
The trigger function I make is:

CREATE OR REPLACE FUNCTION download_counter()
  RETURNS trigger AS
$BODY$
DECLARE
 tmpid INTEGER;
BEGIN
SELECT id INTO tmpid FROM download WHERE songid=NEW.songid AND weeknumber=NEW.weeknumber;
IF NOT FOUND THEN
RETURN NEW;
ELSE
UPDATE download SET counter=counter+1 WHERE id=NEW.id;
RETURN NEW;
END IF;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION download_counter()
  OWNER TO postgres;

And below is my trigger:

CREATE TRIGGER update_or_insert
  BEFORE INSERT
  ON download
  FOR EACH ROW
  EXECUTE PROCEDURE download_counter();

Since I cannot find replace in postgresql ,I think maybe I misused the return type.
Thanks very much for anyone who can give me some advices.

[rski]

You can also use writeable CTE if you use postgres 9.1
See here

http://vibhorkumar.wordpress.com/2011/10/26/upsertmerge-using-writable-cte-in-postgresql-9-1/

[whsdu]

Thanks for your reply.
I would like to integrate the sql into my code,such as:

insert into download (weeknumber, singerid, counter) values ((SELECT EXTRACT(WEEK FROM (SELECT CURRENT_TIMESTA MP))), ?, 1)

so, I can do it with sql on the console, but I have no idea how to integrate the "WITH upsert as" statement into my code.