MySQL的光標不取裏面的程序

Question

我有以下表

專輯：

+----------------+-------------+------+-----+---------+----------------+ 
| 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 |    | 
+----------------+-------------+------+-----+---------+----------------+ 

和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 |    | 
+-----------------+---------+------+-----+---------+----------------+ 

每一個後插入* music_rating *我想更新平均評分在相冊列表中。我有一個觸發器，它調用一個過程。事情是，該過程不起作用，出於某種原因，遊標不從表中獲取數據。 （我單獨調用的程序，以確保它不是觸發演戲了。該表有一對夫婦行了，所以它不是。）

我的程序是非常簡單的，看起來像這樣

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 ; 

我在FETCH cur INTO rating;命令後附加了SELECT rating的遊標結果，它顯示爲空。

Answer 1

您不需要計算avg_rating並將其存儲在albums表中。你可以在飛行計算 -

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