SELECT oder INSERT eine Zeile in einem Befehl

Haben Sie versucht, es zu vereinen?

Bearbeiten - dies erfordert Postgres 9.1:

create table mytable (id serial primary key, other_key varchar not null unique);

WITH new_row AS (
INSERT INTO mytable (other_key)
SELECT 'SOMETHING'
WHERE NOT EXISTS (SELECT * FROM mytable WHERE other_key = 'SOMETHING')
RETURNING *
)
SELECT * FROM new_row
UNION
SELECT * FROM mytable WHERE other_key = 'SOMETHING';

ergibt:

 id | other_key 
----+-----------
  1 | SOMETHING
(1 row)