Desencadenar para actualizar la fila en otra tabla

Suponiendo una estructura de tabla como esta:

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);

Defina los siguientes disparadores:

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 ;

Y debería funcionar así:

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)

Esto está muy simplificado, solo para ilustrar cómo trabajar con activadores. En una aplicación real, tendría más columnas, índices, etc.