Question

I have the following tables

albums:

+----------------+-------------+------+-----+---------+----------------+
| Field          | Type        | Null | Key | Default | Extra          |
+----------------+-------------+------+-----+---------+----------------+
| album_id       | int(11)     | NO   | PRI | NULL    | auto_increment |
| band_id        | int(11)     | YES  | MUL | NULL    |                |
| release_date   | varchar(45) | YES  |     | NULL    |                |
| name           | varchar(45) | YES  |     | NULL    |                |
| format         | varchar(45) | YES  |     | NULL    |                |
| music_genre_id | int(11)     | YES  | MUL | NULL    |                |
| label_id       | int(11)     | YES  | MUL | NULL    |                |
| avg_rating     | float       | YES  |     | NULL    |                |
+----------------+-------------+------+-----+---------+----------------+

and music_ratings

+-----------------+---------+------+-----+---------+----------------+
| Field           | Type    | Null | Key | Default | Extra          |
+-----------------+---------+------+-----+---------+----------------+
| music_rating_id | int(11) | NO   | PRI | NULL    | auto_increment |
| user_id         | int(11) | YES  | MUL | NULL    |                |
| album_id        | int(11) | YES  | MUL | NULL    |                |
| rating          | int(11) | YES  |     | NULL    |                |
+-----------------+---------+------+-----+---------+----------------+

After every insert into the *music_rating* I want to update the average rating in the albums table. I have a trigger for this, which calls a procedure. The thing is, the procedure does not work, for some reason the cursor is not fetching data from the table. (I called the procedure separately to make sure it isn’t the trigger acting up. The tables have a couple of rows already, so it’s not that.)

My procedure is pretty straight forward and looks like this

DELIMITER $$

CREATE PROCEDURE avg_album_calc(IN id_album INT)
BEGIN

    DECLARE done INT DEFAULT 0; 
    DECLARE rating INT; 
    DECLARE cur CURSOR FOR SELECT `rating` FROM `music_ratings` WHERE `album_id`=id_album;
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = 1;

    OPEN cur;   
    SET @ct=0; 
    SET @sm=0;
    REPEAT
        FETCH cur INTO rating;
        IF NOT done
        THEN 
            SET @ct = @ct +1;
            SET @sm = @sm + rating;     
        END IF;

    UNTIL done END REPEAT;
    UPDATE albums SET avg_rating = @sm/@ct WHERE album_id = id_album;
    CLOSE cur;
END$$
DELIMITER ;

I echoed the result of the cursor with a SELECT rating after the FETCH cur INTO rating; command, and it shows up as null.

Answer 1

You do not need to calculate and store avg_rating in the albums table. You can calculate in on the fly –

SELECT a.album_id, a.name, AVG(mr.rating) FROM albums a
  LEFT JOIN music_ratings mr
    ON a.album_id = mr.album_id
GROUP BY a.album_id