个人而言,我会使用触发器来保持的平均等级,因为它会为你的数据库的大小增长更有效率 - 那么你可以简单地做:
select rating from video where video_id = 1;
完整的脚本如下:
drop table if exists video;
create table video
(
video_id int unsigned not null auto_increment primary key,
title 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 video_vote;
create table video_vote
(
video_id int unsigned not null,
user_id int unsigned not null,
score tinyint unsigned not null default 0, -- 0 to 5
primary key (video_id, user_id)
)
engine=innodb;
delimiter #
create trigger video_vote_after_ins_trig after insert on video_vote
for each row
begin
update video set
num_votes = num_votes + 1,
total_score = total_score + new.score,
rating = total_score/num_votes
where
video_id = new.video_id;
end#
delimiter ;
insert into video (title) values ('video 1'),('video 2'), ('video 3');
insert into video_vote (video_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 rating from video where video_id = 1;
问题是什么?你有错误吗?我认为你正在寻找的函数是'IFNULL(exp1,exp2)'。函数ISNULL(exp)只有一个表达式,并返回true或false。 – Galz 2011-03-03 19:59:14