Impede the database writing with a trigger - oracle

Question

I wrote this trigger to impede the database's writing
create or replace trigger spese_autorizzate_trg
before insert or update on spese
for each row
declare
boolean integer := 0;
voceSpesa tipospesa.descrizione%TYPE := NULL;
begin
select
descrizione into voceSpesa
from
tipospesa
where
id = :new.tipospesa
and approvazione = 'n';
if voceSPesa is NULL then
raise_application_error(-20000, 'La spesa '||voceSpesa||' non è rimborsabile');
end if;
end;
If the value of tipospesa is 4 or 5, the writing should be impeded
but when I insert a row like this
insert into spese(id, importo, tipospesa, data) values (4, 20, 3, TO_DATE('15-jul-18', 'DD-MON-RR'))
I have this error
Error report:
SQL Error: ORA-01403: no data found
ORA-06512: at "PARLAMENTO2018.SPESE_AUTORIZZATE_TRG", line 7
ORA-04088: error during execution of trigger
'PARLAMENTO2018.SPESE_AUTORIZZATE_TRG'
01403. 00000 - "no data found"
*Cause:
*Action:
and the writing isn't done. Why?

Answer 1

To me, it seems that you'd have to handle NO_DATA_FOUND, instead of handling possibility that DESCRIZIONE is NULL. Something like this:
create or replace trigger spese_autorizzate_trg
before insert or update on spese
for each row
declare
-- boolean integer := 0; -- Uh, no! Boolean is name of PL/SQL datatype;
-- don't use it as name of a variable
-- Besides, it is never used in your code.
voceSpesa tipospesa.descrizione%TYPE; -- No need to set it to NULL explicitly;
-- it is NULL anyway
begin
select descrizione
into voceSpesa
from tipospesa
where id = :new.tipospesa
and approvazione = 'n';
exception
when no_data_found then
raise_application_error(-20000, 'La spesa '||voceSpesa||' non è rimborsabile');
when too_many_rows then
-- what then? Do you know? Can that select return more than a single row? If so,
-- you should handle it
null;
end;
True, you could save some typing by using select max(descrizione) ..., but that's kind of tricky. If someone else inherits your code, will they know that you used MAX to avoid NO_DATA_FOUND, or whether you intentionally meant to select the largest value of that column? Therefore, I'd say that it is better to actually handle exceptions you expect and avoid any doubt.

Answer 2

The problem is that an INTO clause will fail if no rows are returned from the table, raising no_data_found
MAX or MIN may be used to give you a NULL in case there were no rows.
select
MAX(descrizione) into voceSpesa
from
tipospesa
where
id = :new.tipospesa
and approvazione = 'n';
Do remember that this will also have null in cases where the column descrizione itself is null. But, it purely depends on your requirement how you would like to handle that situation if it ever occurs.