postgresql: Add triggers to the table

1. Trigger Function: inserted into a user data table, if the table data is useless, the field must be empty created_by; created_by the table data must not be empty.
2. New functions during storage;

   CREATE OR REPLACE FUNCTION public.onaddfirst()
    RETURNS trigger
    LANGUAGE plpgsql
   AS $function$
   DECLARE
       total integer;
   BEGIN
       SELECT count(*) INTO total FROM public.user;
       IF total != 0 THEN
           IF NEW.created_by IS NULL THEN
               RAISE EXCEPTION 'created_by cannot be null';
           END IF;
       ELSE
           IF NEW.created_by  is not NULL THEN
               RAISE EXCEPTION 'created_by must be null when you insert first user';
           END IF;
       END IF;
       RETURN NEW;
   END;
   $function$;

    
3. Add Trigger

-- DROP TRIGGER onadd ON public."user";

create trigger onadd before
insert
    on
    public."user" for each row execute procedure onaddfirst();

 

reference

 PostgreSQL study manual (PL / pgSQL procedural language) - Stephen_Liu - blog Park

 PostgreSQL functions (stored procedures) - PostgreSQL ™ Tutorials

 PostgreSQL trigger - Ryan_zheng - blog Park