Getting values from a TABLE using SELECT inside a TRIGGER (Oracle SQL) - oracle

Question

I am working on a Music Collection Database Project and I am stuck on this TRIGGER code. There is an Album table and the task of the trigger is to check if the album being entered already exists in the table or not. And if it does, it should copy the rating which is already present in the table for the album irrespective of the new one entered by the user. (I am using Oracle 11g)
The trigger I managed to get is :
CREATE OR REPLACE TRIGGER album_constraints BEFORE INSERT ON Album
DECLARE
a_id int;
a_title varchar(50);
a_duration varchar(6);
a_number_of_tracks int;
a_recorded date;
a_rating int;
BEGIN
SELECT album_id,
album_title,
album_duration,
album_number_of_tracks,
album_recorded,
album_rating
INTO a_id,
a_title,
a_duration,
a_number_of_tracks,
a_recorded,
a_rating
FROM Album
WHERE album_title = new.album_title and
album_duration = new.album_duration and
album_number_of_tracks = new.album_number_of_tracks and
album_recorded = new.album_recorded and
rownum = 1;
IF new.album_title = a_title and
new.album_duration = a_duration and
new.album_number_of_tracks = a_number_of_tracks and
new.album_recorded = a_recorded
THEN
new.album_rating := a_rating;
END IF;
END;
Can someone please help me achieve this? I am getting multiple errors such as "PL-SQL : Statement ignored"

Answer 1

Actually, there is a way to do what you are trying to do using a single table. It just doesn't involve triggers. Here's a code fragment showing the insert statement:
begin
insert into Album( id, title, duration, number_of_tracks, recorded, rating )
values( v_id, v_title, v_duration, v_number_of_tracks, v_recorded, v_rating );
exception
when DUP_VAL_ON_INDEX then
update Album
set rating = v_rating
where id = v_id;
end;
This assumes ID is the PK and it is something like an ISPN number which uniquely identifies each published album, not a sequential value. If the album does not exist in the table, the Insert statement executes normally. If the album already exists, the Insert statement fails, the exception is caught and the Update is executed instead.
If you really, really want to use a trigger, then front the table with a view and create an "instead of" trigger on the view. To avoid having to make any changes in the app code, name the view Album and the table something else. (My convention is to call the table "Album_". The trailing underscore tells me that no direct access to the table is allowed and there is a view without an underscore where all access takes place. You can use whatever convention you want, of course.)
CREATE OR REPLACE TRIGGER album_constraints
instead of INSERT ON Album
for each row
begin
insert into Album_( id, title, duration, number_of_tracks, recorded, rating )
values( :new.id, :new.title, :new.duration, :new.number_of_tracks, :new.recorded, :new.rating );
exception
when DUP_VAL_ON_INDEX then
update Album_
set rating = :new.rating
where id = :new.id;
end;
Here is where I have to give a hat tip to Sql Server. It allows "instead of" triggers on tables making a "pass thru" view unnecessary.