Trigger for checking values for inserting - plsql

Question

I have two tables that here are involved from two different schemas.
Schema services and table task -- column ID
Schme mona_internal and table officius_unos -- column task
I need trigger when inserting in column task table officius_unos to check does exist inserting value in column id from table task. If exist, to continue inserting, it doesn't exist to raise the error.
Here is the trigger:
CREATE OR REPLACE TRIGGER mona_internal.PROBA_PROBA
BEFORE INSERT ON OFFICIUS_UNOS
FOR EACH ROW
DECLARE
task_provera number(10);
BEGIN
select id into task_provera from servis.task
where id=:new.task;
if (task_provera is null)
then raise_application_error(-20101, 'No task');
else insert into mona_internal.OFFICIUS_UNOS (task) values (:new.task);
end if;
END;
The trigger is compiled, but the problem appears when trying to insert a new value in column task table officius_unos,
it returns me this message
insert into officius_unos (task) values (291504);
Error report -
ORA-00036: maximum number of recursive SQL levels (50) exceeded
ORA-00036: maximum number of recursive SQL levels (50) exceeded
ORA-06512: at "MONA_INTERNAL.PROBA_PROBA", line 5
ORA-04088: error during execution of trigger 'MONA_INTERNAL.PROBA_PROBA'
ORA-06512: at "MONA_INTERNAL.PROBA_PROBA", line 10
And value 291504 exist in table task in column id.
P.S. Also try to solve this problem with check constraint, but there are forbidden subqueries. The solution that I used to overcome my problem is here
Oracle: Using subquery in a trigger

Answer 1

You don't need to insert in an insert trigger.
If the trigger is successful Oracle will continue with the INSERT on its own.
So the immediate solution is to remove the INSERT from the trigger:
CREATE OR REPLACE TRIGGER mona_internal.PROBA_PROBA
BEFORE INSERT ON OFFICIUS_UNOS
FOR EACH ROW
DECLARE
task_provera number(10);
BEGIN
select id
into task_provera
from servis.task
where id=:new.task;
if (task_provera is null) then
raise_application_error(-20101, 'No task');
end if;
// nothing do do here
END;
However the above still isn't correct. If the id doesn't exist in servis.tak the SELECT will throw a NO_DATA_FOUND exception.
One solution to that is to use an aggregate function that will always return one row. If no rows match the WHERE criteria, a NULL value is returned:
CREATE OR REPLACE TRIGGER mona_internal.PROBA_PROBA
BEFORE INSERT ON OFFICIUS_UNOS
FOR EACH ROW
DECLARE
task_provera number(10);
BEGIN
select max(id)
into task_provera
from servis.task
where id=:new.task;
if (task_provera is null) then
raise_application_error(-20101, 'No task');
end if;
// nothing do do here
END;
Or you could explicitely catch the exception:
CREATE OR REPLACE TRIGGER mona_internal.PROBA_PROBA
BEFORE INSERT ON OFFICIUS_UNOS
FOR EACH ROW
DECLARE
task_provera number(10);
BEGIN
select max(id)
into task_provera
from servis.task
where id=:new.task;
if (task_provera is null) then
raise_application_error(-20101, 'No task');
end if;
EXCEPTION
WHEN NOT_DATA_FOUND THEN
raise_application_error(-20101, 'No task');
END;
But the correct approach is to use a foreign key constraint for something like that.
alter table mona_internal.PROBA_PROBA
add constraint fk_proba_task
foreign key (task)
references servis.task (id);
Then you don't need a trigger at all.
This requires that the user mona_internal is not only granted the SELECT privilege on servis.task, but also the references privilege:
To do that, run the following as the SERVIS user:
grant references on task to mona_internal;