Insert trigger if record exists - Oracle - oracle

Question

I have tables:
DEPARTMETS with dep_id(number), dep_name(varchar), manager_id(number) fields.
EMPLOYEES with employee_id(number), name(varchar), salary(number), manager_id(number)
I want to create a trigger which it is responsible for check if exists manager_id into table DEPARTMENTS when data is inserted or updated to table EMPLOYEES
Trigger could be something like:
create or update trigger manager_exists
before insert or update on employees
for each row
begin
if exists **new id** into Departments then
INSERT DATA IN EMPLOYEES
else
"Error: MANAGER_ID doesnt exists in Departments"
end if;
end manager_exists;
But I can't figure out how to create this trigger.
Note:I need it to be a trigger please. HELP!

Answer 1

This is how I understood the question:
create or replace trigger manager_exists
before insert or update on employees
for each row
declare
l_mgr number;
begin
select 1
into l_mgr
from dual
where exists (select null
from departments d
where d.manager_id = :new.manager_id
);
exception
when no_data_found then
raise_application_error(-20000, 'That manager does not exist in DEPARTMENTS');
end manager_exists;
In other words:
check whether MANAGER_ID you're trying to insert into EMPLOYEES table exists in DEPARTMENTS table
though, that doesn't make much sense to me; I'd say that you should check it vice versa - check whether MANAGER_ID you're trying to insert into DEPARTMENTS exists in EMPLOYEES table ... but that's not what you wrote (or I misunderstood what you said)
if so, fine, don't do anything (in a trigger); insert or update statement which caused the trigger to fire will do the job
if not, raise an error