SELECT with INNER JOIN, GROUP BY by and WHERE - mysql

Question

I have a 3 MySQL tables about movies. First one is movie, second is actor and last one is movie_actor_mapping.
CREATE TABLE `movie` (
`movie_id` int(11) NOT NULL AUTO_INCREMENT,
`title` varchar(50) DEFAULT NULL,
`year` varchar(9) DEFAULT NULL,
`rating` float(2,1) DEFAULT NULL,
`runtime` varchar(6) DEFAULT NULL,
`plot` varchar(1500) DEFAULT NULL,
PRIMARY KEY (`movie_id`),
UNIQUE KEY `title` (`title`)
)
CREATE TABLE `actor` (
`actor_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(40) DEFAULT NULL,
PRIMARY KEY (`actor_id`),
UNIQUE KEY `name_en` (`name`)
)
CREATE TABLE `movie_actor_mapping` (
`movie_actor_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`movie_id` int(10) unsigned DEFAULT NULL,
`actor_id` int(10) unsigned DEFAULT NULL,
PRIMARY KEY (`movie_actor_id`),
UNIQUE KEY `movie_actor_id` (`movie_actor_id`)
)
Some movies has NULL in rating and when I do SELECT I have to select all movies which has rating and order them by rating. My SELECT looks like:
SELECT *, group_concat(a.name separator ', ') as actors
FROM movie m INNER JOIN movie_actor_mapping ma
ON m.movie_id=ma.movie_id
INNER JOIN actor a
ON a.actor_id=ta.actor_id
GROUP BY m.movie_id;
Also I wrote a select and order SELECT * FROM movie WHERE rating is not null ORDER BY rating DESC; but I don't understand how to split these queries. I know that if I will not have GROUP BY - I can write my second query after ON but how to do that using GROUP BY?
UPD:
I add example data from my tables. So, for movie it's:
1 | "American Horror Story" | "2011–" | 8.2 | "60 min" | "Both physical..."
A data from actor table:
1 | Evan Rachel Wood
From movie_actor_mapping:
1 | 21 | 1

Answer 1

Maybe I don't understand the question, because it seems too simple, but this query:
SELECT m.*, group_concat(a.name separator ', ') as actors
FROM movie m
INNER JOIN movie_actor_mapping ma ON m.movie_id = ma.movie_id
INNER JOIN actor a ON a.actor_id = ma.actor_id
WHERE m.rating IS NOT NULL
GROUP BY m.movie_id
ORDER BY m.rating DESC
...is selecting movies with rating, groups actors and orders them by rating

Answer 2

Try:
SELECT *, group_concat(a.name order by rating desc separator ', ' ) as actors
FROM movie m INNER JOIN movie_actor_mapping ma
ON m.movie_id=ma.movie_id
INNER JOIN actor a
ON a.actor_id=ta.actor_id
WHERE rating is not null
GROUP BY m.movie_id;

Answer 3

When you use GROUP BY, you need to have some aggregation action on those returned values that are not used for grouping (for instance, COUNT, MAX, MIN, etc.).
If what you want is to get the list of movies where all records of a specific movie are together, you can use the ORDER BY clause. In your case, it could be movie_id.