Not null constraint using triggers in SQL - sql

Question

I want to implement a not-null constraint on an attribute using a trigger.
Here's my code:
create table mytable2(id int);
create or replace function p_fn() returns trigger as $prim_key$
begin
if (tg_op='insert') then
if (id is null) then
raise notice 'ID cannot be null';
return null;
end if;
return new;
end if;
end;
$prim_key$ language plpgsql;
create trigger prim_key
before insert on mytable2
for each row execute procedure p_fn();
But I get an error saying "control reached end of trigger procedure without RETURN" whenever I try to insert a null value. I tried placing the "return new" statement in the inner IF, but it still gave me the same error. What am I doing wrong?

Answer 1

The immediate cause of the problem is that PostgreSQL string comparisons are case sensitive. INSERT is not the same as insert. Try:
IF tg_op = 'INSERT' THEN
Advice
You're only raising a notice. This allows flow of control to continue to the next line in the procedure. You should generally RAISE EXCEPTION to abort execution and roll the transaction back. See RAISE. As it stands, the trigger will cause inserts that do not satisfy the requirement to silently fail, which is generally not what you want.
Additionally, your triggers should usually end with a RAISE EXCEPTION if they're always supposed to return before end of function. That would've helped you see what was going wrong sooner.

Answer 2

I'd add just a couple of things to the good suggestions already made:
make sure you handle the UPDATE case as well
RAISE using the proper error condition. I've given a basic example below - for more formatting options see the docs here.
for clarity, I like to include at least the table name in my trigger name
CREATE OR REPLACE FUNCTION fn_validate_id_trigger() RETURNS TRIGGER AS
$BODY$
BEGIN
IF (TG_OP IN ('INSERT', 'UPDATE')) THEN
IF (NEW.id IS NULL) THEN
RAISE not_null_violation;
END IF;
END IF;
RETURN NULL;
END;
$BODY$
LANGUAGE plpgsql;
CREATE CONSTRAINT TRIGGER tr_mytable_validate_id
AFTER INSERT OR UPDATE
ON mytable2
FOR EACH ROW EXECUTE PROCEDURE fn_validate_id_trigger();
Update: this is now a CONSTRAINT trigger and fires AFTER insert or update. In first edit, I presented a column-specific trigger (UPDATE OF id). This was problematic as it would not fire if another trigger executed on the table changed column 'id' to null.
Again, this isn't the most efficient way to handle constraints but it's good to know.