Writing complicated row level trigger In Oracle - oracle

Question

I have to create a row level trigger on a table called Movies that will compute the five greatest movie ratings from this Movies table and insert a new table called TopMovies with these movies. Each time I put a new rating in the Movies table this trigger fires. I am really struggling to understand how to go about this.
The Movies table has the following attributes (Movie, Director, Rating)
The Top Movies table will have (Rating)
So far I have just the following code. I really am lost as to how to make it so that once I have inserted at least 5 ratings into the movies table, the TopMovies table will delete the lowest rated movie and re-populate it with a new movie. I am not asking for the answer but any help to go in the right direction
Create of replace Trigger top_trigger
After insert or update on Movies
For each row
Begin
insert into TopMovies values (:new.rating),
End

Answer 1

You can use the following triggers. But be aware that concurrent requests may change the expected behavior when you use the triggers.
Trigger solution:
CREATE OR REPLACE TRIGGER TOP_TRIGGER AFTER
INSERT OR UPDATE ON MOVIES
FOR EACH ROW
DECLARE
LV_CNT NUMBER := 0;
LV_RATING TOPMOVIES.RATING%TYPE;
BEGIN
SELECT COUNT(1), MIN(RATING)
INTO LV_CNT, LV_RATING
FROM TOPMOVIES;
IF LV_CNT >= 5 AND :NEW.RATING > LV_RATING THEN
DELETE FROM TOPMOVIES WHERE RATING = LV_RATING;
END IF;
IF LV_CNT < 5 OR ( LV_CNT >= 5 AND :NEW.RATING > LV_RATING ) THEN
INSERT INTO TOPMOVIES VALUES ( :NEW.RATING );
END IF;
END;
/
The better solution is to use the view.
VIEW solution:
CREATE OR REPLACE VIEW TOPMOVIES_VW AS
SELECT * FROM
(
SELECT T.*,
DENSE_RANK() OVER( ORDER BY RATING DESC) AS RANK#
FROM MOVIES T
)
WHERE RANK# <= 5;
Cheers!!

Answer 2

You may try below code -
Create of replace Trigger top_trigger
After insert or update on Movies
Declare
v_movie_count number:= 0;
Begin
select count(*)
into v_movie_count
from Movies;
if v_movie_count >= 5 then
delete from TopMovies;
INSERT INTO TopMovies
SELECT *
FROM (SELECT Rating, ROWNUM RN
FROM Movies
ORDER BY Rating)
WHERE RN <= 5;
/* You can use below statement if you are using 12C or higher*/
/*INSERT INTO TopMovies
SELECT Rating, ROWNUM RN
FROM Movies
ORDER BY Rating DESC
FETCH FIRST 5 ROWS ONLY;*/
end if;
End