mysql中的AVG和ISNULL

Question

我需要实现这样的目标：

SELECT ISNULL(AVG(rating),0) FROM videorating vr WHERE vr.VideoId=11229;

如果平均值为空/空，那么我应该得到 0 作为评级。

我正在尝试优化这个查询

SELECT CASE ISNULL(AVG(rating)) WHEN 0 THEN AVG(rating) ELSE 0 END AS rating FROM videorating WHERE videoID=11229;

Answer 1

就我个人而言，我会使用触发器来保持平均评级，因为随着数据库大小的增长，它会更加有效 - 然后你可以简单地执行以下操作：

select rating from video where video_id = 1;

完整脚本如下：

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;