Postgres trigger function

Question

I need help in Postgres triggers.

I have table with 2 columns:

sold boolean;
id_shop int;

I

Answer 1

First of all you need a before trigger if you want to change a value of the row being updated (or inserted)

Secondly you don't need to "update" the table, just assign the new value to the NEW row:

create or replace function pardota_masina_veikals() 
RETURNS trigger 
AS 
$pardota_masina$
begin
  IF NEW.sold=true THEN
    NEW.id_shop = NULL;
 END IF;
RETURN NEW;
END;
$pardota_masina$ 
LANGUAGE plpgsql;

CREATE TRIGGER pardota_masina_nevar_but_veikala 
   BEFORE INSERT OR UPDATE ON masinas 
   FOR EACH ROW EXECUTE PROCEDURE pardota_masina_veikals();