ecco un semplice esempio di MySQL:
drop table if exists image;
create table image
(
image_id int unsigned not null auto_increment primary key,
caption varchar(255) not null,
num_votes int unsigned not null default 0,
total_score int unsigned not null default 0,
rating decimal(8,2) not null default 0
)
engine = innodb;
drop table if exists image_vote;
create table image_vote
(
image_id int unsigned not null,
user_id int unsigned not null,
score tinyint unsigned not null default 0,
primary key (image_id, user_id)
)
engine=innodb;
delimiter #
create trigger image_vote_after_ins_trig after insert on image_vote
for each row
begin
update image set
num_votes = num_votes + 1,
total_score = total_score + new.score,
rating = total_score / num_votes
where
image_id = new.image_id;
end#
delimiter ;
insert into image (caption) values ('image 1'),('image 2'), ('image 3');
insert into image_vote (image_id, user_id, score) values
(1,1,5),(1,2,4),(1,3,3),(1,4,2),(1,5,1),
(2,1,2),(2,2,1),(2,3,4),
(3,1,4),(3,5,2);
select * from image;
select * from image_vote;
