PLSQL trigger not working 0 rows inserted - oracle

Question

Whatever I do to the trigger always returns "0 rows inserted".
It is like it can't find the new values after inserting them.
After adding the exception it return no_data_found and i don't know why.
before insert or update of rent_date, return_date on rent
for each row
declare
pragma AUTONOMOUS_TRANSACTION;
v_rentDate date;
v_returnDate date;
begin
select rent_date
into v_rentDate
from rent
where rent_date = :new.rent_date;
select return_date
into v_returnDate
from rent
where return_date = :new.return_date;
if v_returnDate < v_rentDate then
raise_application_error(-20158, 'Return date must be after the rent date');
else
dbms_output.put_line('TEST');
end if;
exception when no_data_found then raise_application_error(-20157, 'No data found');
end;
/
insert into rent values (82,sysdate-5,101,sysdate,sysdate+5,100);
--0 rows inserted

Answer 1

It appears that you're doing it the wrong way. Here's why:
you are trying to select values from a table you're currently inserting into (or updating existing values)
Oracle complains that it can't do that because the table is mutating
in order to "fix" it, you used pragma autonomous_transaction which isolates trigger code from the main transaction
You shouldn't use that pragma for such a purpose. Lucky you, trigger can be rewritten in a simpler manner, the one that doesn't cause the mutating table error. As you want to compare rent_date and return_date, do it directly. Here's an example (see line #5):
SQL> create table rent
2 (id number,
3 rent_date date,
4 return_date date
5 );
Table created.
SQL> create or replace trigger trg_biu_rent
2 before insert or update on rent
3 for each row
4 begin
5 if :new.return_date < :new.rent_date then
6 raise_application_error (-20158, 'Return date must be after the rent date');
7 end if;
8 end;
9 /
Trigger created.
Testing:
SQL> -- This will fail
SQL> insert into rent (id, rent_date, return_date) values
2 (1, date '2019-05-25', date '2019-04-10');
insert into rent (id, rent_date, return_date) values
*
ERROR at line 1:
ORA-20158: Return date must be after the rent date
ORA-06512: at "SCOTT.TRG_BIU_RENT", line 3
ORA-04088: error during execution of trigger 'SCOTT.TRG_BIU_RENT'
SQL> -- This is OK
SQL> insert into rent (id, rent_date, return_date) values
2 (1, date '2019-03-28', date '2019-10-20');
1 row created.
SQL>