Computed Column as Average from Other Data in Another Table - mysql

Question

I am creating a simple movie website that allows users to browse through movie titles, and rate movies with a 5 point rating system. I am using XAMPP, and phpAdmin to store my database through the SQL language. I have the following table below that stores ratings:
CREATE TABLE `movie_ratings` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`movie_id` int(11) DEFAULT NULL,
`rating` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
I have the following table below that stores movies:
CREATE TABLE `movies` (
`movie_title` varchar(255) NOT NULL,
`movie_id` int(100) NOT NULL,
`genre` text NOT NULL,
`release_date` text NOT NULL,
`price` int(100) NOT NULL,
`year` year(4) NOT NULL,
`description` text NOT NULL,
`movie_image` text NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
I want to add a column, average_rating to my movies table that basically takes all rating values from movie_ratings from a specific movie_id and averages them and then stores the value into this new average_rating column within the movies table.
I believe I need a computed column, but I only know how to do this via columns from the same table, here I am using another table. I read that a view could be possible too... Would prefer to store the average rating in movies though as a new column.
So I created a VIEW but it's only displaying one movie and averaging.
CREATE VIEW movie_ratings_view AS
SELECT b.movie_id, b.movie_title, b.movie_image, b.price, AVG(br.rating)
AS avgRating FROM movies b INNER JOIN movie_ratings br ON
b.movie_id = br.movie_id;

Answer 1

You need a group by clause
CREATE VIEW movie_ratings_view
AS
SELECT
b.movie_id
, b.movie_title
, b.movie_image
, b.price
, AVG(br.rating) AS avgRating
FROM movies b
INNER JOIN movie_ratings br ON b.movie_id = br.movie_id
GROUP BY
b.movie_id
, b.movie_title
, b.movie_image
, b.price
;
An alternative (if/when subqueries are allowed in the from clause of a view by MySQL):
CREATE VIEW movie_ratings_view
AS
SELECT
m.movie_id
, m.movie_title
, m.movie_image
, m.price
, COALESCE(mr.numRatings, 0) AS numRatings
, COALESCE(mr.avgRating, 0) AS avgRating
FROM movies m
LEFT JOIN (
SELECT
movie_id
, COUNT(rating) AS numRatings
, AVG(rating) AS avgRating
FROM movie_ratings
GROUP BY movie_id) mr ON m.movie_id = mr.movie_id
;

Answer 2

Probably the most efficient code for a view is a correlated subquery:
CREATE VIEW movie_ratings_view AS
SELECT m.movie_id, m.movie_title, m.movie_image, m.price
(SELECT AVG(mr.rating)
FROM movie_ratings mr
WHERE mr.movie_id = m.movie_id
) as avgRating
FROM movies m;
This can make use of an index on movie_ratings(movie_id, rating). And it doesn't have the outer GROUP BY which can be expensive.
It is slightly different because this will return movies with no ratings -- but that might even be desirable.