Trigger in Oracle: No data found - oracle

Question

Please check my code, I seldom use trigger with AFTER clause, thanks so much:
set serveroutput on;
create or replace trigger tvideo_2 after insert on video for each row
declare
pragma autonomous_transaction;
v_title video.title%type; /* Declare a variable to check title that contain '18+' */
begin
select title into v_title from video where video.videoid=:new.videoid;
if v_title like '%18+%' then
update video
set age=18
where videoid=:new.videoid;
dbms_output.put_line('Video '||:new.videoid||' has been updated age to 18+');
else
dbms_output.put_line('Video '||:new.videoid||' is not 18+!');
end if;
end;
/
insert into video values('V5', '18+', 240, 19);
VIDEO properties: (videoid, title, duration, age)

Answer 1

Using pragma autonomous_transaction means that the trigger cannot see the row that has just been inserted and which cause it to be fired.
When you removed the primary key and pre-inserted a record with the same ID, it's that row that will be updated by your trigger, not the new one you are inserting - so it still won't do what you want, and duplicating the PK value isn't sensible anyway.
If you have to do this as an after insert trigger then you can remove the for each row, which avoids the mutating table issue you're presumably trying to avoid, but then you have to query the whole table to find rows to update:
create or replace trigger tvideo_2 after insert on video
begin
update video
set age = 18
where title like '%18+%'
and age != 18;
end;
/
Or if you want to print a message:
create or replace trigger tvideo_2 after insert on video
begin
for rec in (
select videoid, title
from video
where title like '%18+%'
and age != 18
)
loop
update video
set age = 18
where video.videoid = rec.videoid;
dbms_output.put_line('Video '||rec.videoid||' has been updated age to 18+');
end loop;
end;
/
But using dbms_output in a trigger, or really in any stored PL/SQL, isn't a good idea - if the client session that does the insert doesn't look at the output buffer than that message is lost, and it's likely there will nothing (or nobody) to see it anyway.
You could also use an explicit cursor with for update and then update .. where current of.
Either way you're having to query the whole table, which isn't very efficient - you're checking all existing rows every time you insert a new one, instead of just checking that single new value. A trigger isn't really suitable to do that much work - even if it only actually updates a single row (or none!) it has to query everything for every insert. That isn't going to scale very well.
There are other way to avoid the mutating table issue but anything is going to be a hack. The sensible way to do this is with a before-insert row level trigger:
create or replace trigger tvideo_2 before insert on video for each row
begin
if :new.title like '%18+%' then
:new.age := 18;
dbms_output.put_line('Video '||:new.videoid||' has been updated age to 18+');
else
dbms_output.put_line('Video '||:new.videoid||' is not 18+!');
end if;
end;
/
(with the same caveat about using dbms_output) but that seems to be forbidden for some reason.