Question

这是一个select语句，用于计算数据库中歌曲的平均评分

SELECT *, (tab.rating_sum/tab.rating_count) as rating_average FROM
(SELECT song_id, COUNT(rating) rating_count, SUM(rating) rating_sum FROM ratings
GROUP BY song_id) tab

INNER JOIN SONGS WHERE SONGS.id = tab.song_id

当在RATINGS中插入新评级时，我需要将RATINGS表中的rating_average列插入表SONGS。我还需要确保它进入正确的song_id列。如何使用触发器执行此操作？我一直在撕开我的头发。

这是我能想到的最好的，但我不知道我在使用触发器做什么：

CREATE TRIGGER rating_trig ON ratings FOR INSERT AS
BEGIN
UPDATE SONGS SET
rating = SELECT (tab.rating_sum/tab.rating_count) as rating_average FROM
(SELECT song_id, COUNT(rating) rating_count, SUM(rating) rating_sum FROM ratings
GROUP BY song_id) tab

INNER JOIN SONGS WHERE SONGS.id = tab.song_id
END

表格布局：

评分| song_id，评分，用户名歌曲| id，rating，song_name

Answer 1

假设像这样的表结构：

use test;

create table song(
  song_id integer,
  rating_avg double,
  rating_sum integer,
  rating_count integer);

create table rating(
  song_id integer,
  user_id integer,
  rating integer);

定义以下触发器：

delimiter $$

create trigger bi_song before insert on test.song
for each row
begin
  set NEW.rating_sum = 0;
  set NEW.rating_count = 0;
  set NEW.rating_avg = NULL;
end
$$

create trigger ai_rating after insert on test.rating
for each row
begin
  update song set
    rating_sum = rating_sum + NEW.rating,
    rating_count = rating_count + 1,
    rating_avg = rating_sum / rating_count
    where song_id = NEW.song_id;
end
$$
delimiter ;

它应该像这样工作：

mysql> insert into song(song_id) values (1);
Query OK, 1 row affected (0.06 sec)

mysql> select * from song;
+---------+------------+------------+--------------+
| song_id | rating_avg | rating_sum | rating_count |
+---------+------------+------------+--------------+
|       1 |       NULL |          0 |            0 |
+---------+------------+------------+--------------+
1 row in set (0.00 sec)

mysql> insert into rating(song_id, user_id, rating) values (1, 1000, 5);
Query OK, 1 row affected (0.05 sec)

mysql> select * from song;
+---------+------------+------------+--------------+
| song_id | rating_avg | rating_sum | rating_count |
+---------+------------+------------+--------------+
|       1 |          5 |          5 |            1 |
+---------+------------+------------+--------------+
1 row in set (0.00 sec)

mysql> insert into rating(song_id, user_id, rating) values (1, 1001, 7);
Query OK, 1 row affected (0.05 sec)

mysql> select * from song;
+---------+------------+------------+--------------+
| song_id | rating_avg | rating_sum | rating_count |
+---------+------------+------------+--------------+
|       1 |          6 |         12 |            2 |
+---------+------------+------------+--------------+
1 row in set (0.00 sec)

这是非常简化的，只是为了说明如何使用触发器。在实际应用程序中，您将拥有更多列，索引等。

触发更新另一个表中的行

1 个答案: