do $$
declare
  selected_film film%rowtype;
  input_film_id film.film_id%type := 0;
begin  

  select * from film
  into selected_film
  where film_id = input_film_id;
  
  if not found then
     raise notice'The film % could not be found', 
	    input_film_id;
  end if;
end $$
Code language: PostgreSQL SQL dialect and PL/pgSQL (pgsql)