CREATE FUNCTION create_user(user_name character varying, user_mail character varying, user_hash character varying) returns BOOLEAN
  language plpgsql
AS
$$
DECLARE
  l_user_count INTEGER;
BEGIN
  SELECT COUNT(1) INTO l_user_count FROM users u WHERE u.mail = user_mail;
  IF l_user_count = 0 THEN
    INSERT INTO users(name, mail, hash) VALUES(user_name, user_mail, user_hash);
  RETURN TRUE;
  END IF;
  RETURN FALSE;
END;
$$;

ALTER FUNCTION create_user(VARCHAR, VARCHAR, VARCHAR) owner TO postgres;